Showing papers in "International Journal of Heat and Mass Transfer in 2009"
TL;DR: In this paper, the authors summarized the important published articles on the enhancement of the forced convection heat transfer with nanofluids, including simulations, simulations, and experimental results.
Abstract: Nanofluids are considered to offer important advantages over conventional heat transfer fluids. Over a decade ago, researchers focused on measuring and modeling the effective thermal conductivity and viscosity of nanofluids. Recently important theoretical and experimental research works on convective heat transfer appeared in the open literatures on the enhancement of heat transfer using suspensions of nanometer-sized solid particle materials, metallic or nonmetallic in base heat transfer fluids. The purpose of this review article is to summarize the important published articles on the enhancement of the forced convection heat transfer with nanofluids.
1,738 citations
TL;DR: In this article, the Cheng-Minkowycz problem of natural convection past a vertical plate, in a porous medium saturated by a nanofluid, is studied analytically.
Abstract: The Cheng–Minkowycz problem of natural convection past a vertical plate, in a porous medium saturated by a nanofluid, is studied analytically. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. For the porous medium the Darcy model is employed. A similarity solution is presented. This solution depends on a Lewis number Le, a buoyancy-ratio number Nr, a Brownian motion number Nb, and a thermophoresis number Nt. The dependency of the Nusslelt number on these four parameters is investigated.
760 citations
TL;DR: In this paper, a model was developed to account for the thermal conductivity of nanofluids as a function of temperature, particle volumetric concentration, the properties of nanoparticles, and the base fluid, which agrees well with the experimental data.
Abstract: Experimental investigations have been carried out for determining the thermal conductivity of three nanofluids containing aluminum oxide, copper oxide and zinc oxide nanoparticles dispersed in a base fluid of 60:40 (by mass) ethylene glycol and water mixture. Particle volumetric concentration tested was up to 10% and the temperature range of the experiments was from 298 to 363 K. The results show an increase in the thermal conductivity of nanofluids compared to the base fluids with an increasing volumetric concentration of nanoparticles. The thermal conductivity also increases substantially with an increase in temperature. Several existing models for thermal conductivity were compared with the experimental data obtained from these nanofluids, and they do not exhibit good agreement. Therefore, a model was developed, which is a refinement of an existing model, which incorporates the classical Maxwell model and the Brownian motion effect to account for the thermal conductivity of nanofluids as a function of temperature, particle volumetric concentration, the properties of nanoparticles, and the base fluid, which agrees well with the experimental data.
750 citations
TL;DR: In this article, the authors measured the pressure drop and convective heat transfer coefficient of water-based Al2O3 nanofluids flowing through a uniformly heated circular tube in the fully developed laminar flow regime.
Abstract: We have measured the pressure drop and convective heat transfer coefficient of water-based Al2O3 nanofluids flowing through a uniformly heated circular tube in the fully developed laminar flow regime. The experimental results show that the data for nanofluid friction factor show a good agreement with analytical predictions from the Darcy’s equation for single-phase flow. However, the convective heat transfer coefficient of the nanofluids increases by up to 8% at a concentration of 0.3 vol% compared with that of pure water and this enhancement cannot be predicted by the Shah equation. Furthermore, the experimental results show that the convective heat transfer coefficient enhancement exceeds, by a large margin, the thermal conductivity enhancement. Therefore, we have discussed the various effects of thermal conductivities under static and dynamic conditions, energy transfer by nanoparticle dispersion, nanoparticle migration due to viscosity gradient, non-uniform shear rate, Brownian diffusion and thermophoresis on the remarkable enhancement of the convective heat transfer coefficient of nanofluids. Based on scale analysis and numerical solutions, we have shown, for the first time, the flattening of velocity profile, induced from large gradients in bulk properties such as nanoparticle concentration, thermal conductivity and viscosity. We propose that this flattening of velocity profile is a possible mechanism for the convective heat transfer coefficient enhancement exceeding the thermal conductivity enhancement.
573 citations
TL;DR: In this paper, the effect of particle size on convective heat transfer in laminar developing region was evaluated with alumina-water nanofluids in tube flow with constant heat flux.
Abstract: An experimental investigation on the convective heat transfer characteristics in the developing region of tube flow with constant heat flux is carried out with alumina–water nanofluids. The primary objective is to evaluate the effect of particle size on convective heat transfer in laminar developing region. Two particle sizes were used, one with average particle size off 45 nm and the other with 150 nm. It was observed that both nanofluids showed higher heat transfer characteristics than the base fluid and the nanofluid with 45 nm particles showed higher heat transfer coefficient than that with 150 nm particles. It was also observed that in the developing region, the heat transfer coefficients show higher enhancement than in the developed region. Based on the experimental results a correlation for heat transfer in the developing region has been proposed for the present range of nanofluids.
528 citations
TL;DR: In this paper, heat transfer and viscous pressure loss were investigated for alumina-water and zirconia-water nanofluids in a flow loop with a vertical heated tube.
Abstract: Laminar convective heat transfer and viscous pressure loss were investigated for alumina–water and zirconia–water nanofluids in a flow loop with a vertical heated tube. The heat transfer coefficients in the entrance region and in the fully developed region are found to increase by 17% and 27%, respectively, for alumina–water nanofluid at 6 vol % with respect to pure water. The zirconia–water nanofluid heat transfer coefficient increases by approximately 2% in the entrance region and 3% in the fully developed region at 1.32 vol %. The measured pressure loss for the nanofluids is in general much higher than for pure water. However, both the measured nanofluid heat transfer coefficient and pressure loss are in good agreement with the traditional model predictions for laminar flow, provided that the loading- and temperature-dependent thermophysical properties of the nanofluids are utilized in the evaluation of the dimensionless numbers. In other words, no abnormal heat transfer enhancement or pressure loss was observed within measurement errors.
472 citations
TL;DR: In this article, the effect of dispersing energy (ultrasonication) on viscosity, thermal conductivity, and the laminar convective heat transfer was studied.
Abstract: Four samples of 1 wt% multi-walled carbon nanotube-based (MWCNT) aqueous nanofluids prepared via ultrasonication were thermally characterized. Direct imaging was done using a newly developed wet-TEM technique to assess the dispersion state of carbon nanotubes (CNT) in suspension. The effect of dispersing energy (ultrasonication) on viscosity, thermal conductivity, and the laminar convective heat transfer was studied. Results indicate that thermal conductivity and heat transfer enhancement increased until an optimum ultrasonication time was reached, and decreased on further ultrasonication. The suspensions exhibited a shear thinning behavior, which followed the Power Law viscosity model. The maximum enhancements in thermal conductivity and convective heat transfer were found to be 20% and 32%, respectively. The thermal conductivity enhancement increased considerably at temperatures greater than 24 °C. The enhancement in convective heat transfer was found to increase with axial distance. A number of mechanisms related to boundary layer thickness, micro-convective effect, particle rearrangement, possible induced convective effects due to temperature and viscosity variations in the radial direction, and the non-Newtonian nature of the samples are discussed.
472 citations
TL;DR: In this paper, an experimental study on the forced convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2 vol.% TiO2 nanoparticles was performed.
Abstract: This article reports an experimental study on the forced convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2 vol.% TiO2 nanoparticles. The heat transfer coefficient and friction factor of the TiO2–water nanofluid flowing in a horizontal double-tube counter flow heat exchanger under turbulent flow conditions are investigated. The Degussa P25 TiO2 nanoparticles of about 21 nm diameter are used in the present study. The results show that the convective heat transfer coefficient of nanofluid is slightly higher than that of the base liquid by about 6–11%. The heat transfer coefficient of the nanofluid increases with an increase in the mass flow rate of the hot water and nanofluid, and increases with a decrease in the nanofluid temperature, and the temperature of the heating fluid has no significant effect on the heat transfer coefficient of the nanofluid. It is also seen that the Gnielinski equation failed to predict the heat transfer coefficient of the nanofluid. Finally, the use of the nanofluid has a little penalty in pressure drop.
458 citations
TL;DR: In this article, the onset of convection in a horizontal layer of a porous medium saturated by a nanofluid is studied analytically, and it is found that the critical thermal Rayleigh number can be reduced or increased by a substantial amount depending on whether the basic nanoparticle distribution is top-heavy or bottom-heavy, by the presence of the nanoparticles.
Abstract: The onset of convection in a horizontal layer of a porous medium saturated by a nanofluid is studied analytically. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. The analysis reveals that for a typical nanofluid (with large Lewis number) the prime effect of the nanofluids is via a buoyancy effect coupled with the conservation of nanoparticles, the contribution of nanoparticles to the thermal energy equation being a second-order effect. It is found that the critical thermal Rayleigh number can be reduced or increased by a substantial amount, depending on whether the basic nanoparticle distribution is top-heavy or bottom-heavy, by the presence of the nanoparticles. Oscillatory instability is possible in the case of a bottom-heavy nanoparticle distribution.
434 citations
TL;DR: In this paper, the influence of surface wettability on nucleate boiling heat transfer was investigated and a new approach of nucleation mechanism was established to clarify the nexus between the surface wetability and the nucleate heating transfer.
Abstract: Experiments were performed to highlight the influence of surface wettability on nucleate boiling heat transfer. Nanocoating techniques were used to vary the water contact angle from 20° to 110° by modifying nanoscale surface topography and chemistry. The bubble growth was recorded by a high speed video camera to enable a better understanding of the surface wettability effects on nucleation mechanism. For hydrophilic (wetted) surfaces, it was found that a greater surface wettability increases the vapour bubble departure radius and reduces the bubble emission frequency. Moreover, lower superheat is required for the initial growth of bubbles on hydrophobic (unwetted) surfaces. However, the bubble in contact with the hydrophobic surface cannot detach from the wall and have a curvature radius increasing with time. At higher heat flux, the bubble spreads over the surface and coalesces with bubbles formed at other sites, causing a large area of the surface to become vapour blanketed. The best heat transfer coefficient is obtained with the surface which had a water contact angle close to either 0° or 90°. A new approach of nucleation mechanism is established to clarify the nexus between the surface wettability and the nucleate boiling heat transfer.
397 citations
TL;DR: The main features of the problems addressed with GAs including the modeling, number of variables, and GA settings are presented, useful for future use of GAs in heat transfer.
Abstract: This review presents when and how Genetic Algorithms (GAs) have been used over the last 15 years in the field of heat transfer. GAs are an optimization tool based on Darwinian evolution. They have been developed in the 1970s, but their utilization in heat transfer problems is more recent. In particular, the last couple of years have seen a sharp increase of interest in GAs for heat transfer related optimization problems. Three main families of heat transfer problems using GAs have been identified: (i) thermal systems design problems, (ii) inverse heat transfer problems, and (iii) development of heat transfer correlations. We present here the main features of the problems addressed with GAs including the modeling, number of variables, and GA settings. This information is useful for future use of GAs in heat transfer. Future possibilities and accomplishments of GAs in heat transfer are also drawn.
TL;DR: In this article, an experimental and computational investigation directed at understanding the role of buoyancy-driven convection during constrained melting of phase change materials (PCM) inside a spherical capsule is reported.
Abstract: An experimental and computational investigation directed at understanding the role of buoyancy-driven convection during constrained melting of phase change materials (PCM) inside a spherical capsule is reported. The computations are based on an iterative, finite-volume numerical procedure that incorporates a single-domain enthalpy formulation for simulation of the phase change phenomenon. A Darcy’s Law-type porous media treatment links the effect of phase change on convection. Paraffin wax n-octadecane was constrained during melting inside a transparent glass sphere through the use of thermocouples installed inside the sphere. The melting phase front and melting fraction of the PCM are analyzed and compared with numerical solution obtained from the CFD code Fluent. Following a short period of symmetric melting due to prominence of diffusion, expedited phase change in the top region of the sphere and a wavy surface at the bottom of the PCM are observed. The computational predictions point to the strong thermal stratification in the upper half of the sphere that results from rising of the molten liquid along the inner surface of the sphere thus displacing the colder fluid. The waviness and excessive melting of the bottom of the PCM is shown to be underestimated by the experimental observation. This discrepancy is linked to the use of a support structure to hold the sphere. Measured temperature data and computational results near the bottom indicate the establishment of an unstable fluid layer that promotes chaotic fluctuations and is responsible for waviness of the bottom of the PCM. On the other hand, the comparison between the measured and computed temperatures in the top half of the sphere show the stable nature of the molten liquid layer. The computational results start to deviate from the thermocouple readings as one moves lower from the top of the sphere. This delay in predicting the melting instant is linked to the thermal stratification within the “constant temperature bath” that encloses the capsule.
TL;DR: In this article, an integrated microsystem consisting of a single microchannel on one side, and two localized heaters and five polysilicon temperature sensors along the channel on the other side were fabricated.
Abstract: Convective heat transfer coefficient and friction factor of nanofluids in rectangular microchannels were measured. An integrated microsystem consisting of a single microchannel on one side, and two localized heaters and five polysilicon temperature sensors along the channel on the other side were fabricated. Aluminum dioxide (Al 2 O 3 ) with diameter of 170 nm nanofluids with various particle volume fractions were used in experiments to investigate the effect of the volume fraction of the nanoparticles to the convective heat transfer and fluid flow in microchannels. The convective heat transfer coefficient of the Al 2 O 3 nanofluid in laminar flow regime was measured to be increased up to 32% compared to the distilled water at a volume fraction of 1.8 volume percent without major friction loss. The Nusselt number measured increases with increasing the Reynolds number in laminar flow regime. The measured Nusselt number which turned out to be less than 0.5 was successfully correlated with Reynolds number and Prandtl number based on the thermal conductivity of nanofluids.
TL;DR: In this article, a composite correlation is developed from a database of 3899 data points from 14 studies in the literature covering 12 different wetting and non-wetting fluids, hydraulic diameters ranging from 016 to 292mm, and confinement numbers from 03 to 40.
Abstract: Recent reviews of flow boiling heat transfer in small tubes and channels have highlighted the need for predictive correlations that are applicable over a wide range of parameters and across different studies A composite correlation is developed in the present work which includes nucleate boiling and convective heat transfer terms while accounting for the effect of bubble confinement in small channels The correlation is developed from a database of 3899 data points from 14 studies in the literature covering 12 different wetting and non-wetting fluids, hydraulic diameters ranging from 016 to 292 mm, and confinement numbers from 03 to 40 The mass fluxes included in the database range from 20 to 3000 kg m −2 s −1 , the heat fluxes from 04 to 115 W cm −2 , the vapor qualities from 0 to 1, and the saturation temperatures from −194 to 97 °C While some of the data sets show opposing trends with respect to some parameters, a mean absolute error of less than 30% is achieved with the proposed correlation
TL;DR: In this paper, the nucleate pool boiling heat transfer of a refrigerant-based nanofluid was investigated at different nanoparticle concentrations and pressures, and the results indicated that the boiling temperature of nucleate pools deteriorated with increasing particle concentrations, especially at high heat flux.
Abstract: Nucleate pool boiling heat transfer of a refrigerant-based-nanofluid was investigated at different nanoparticle concentrations and pressures. TiO 2 nanoparticles were mixed with the refrigerant HCFC 141b at 0.01, 0.03 and 0.05 vol%. The experiment was performed using a cylindrical copper tube as a boiling surface. Pool boiling experiments of nanofluid were conducted and compared with that of the base refrigerant. The results indicate that the nucleate pool boiling heat transfer deteriorated with increasing particle concentrations, especially at high heat fluxes. At 0.05 vol%, the boiling heat transfer curves were suppressed. At high pressures of 400 and 500 kPa, the boiling heat transfer coefficient at a specific excess temperature was almost the same.
TL;DR: In this article, a model for morphology of liquid phase across multiple porous layers by use of both continuum and breakthrough (percolation) treatments was developed, and the results of this model showed the liquid morphologies deteriorate the efficiency of electrochemical reactions in CL and increase the water saturation in GDL.
Abstract: We have used environmental scanning electron microscope to observe vapor condensation and liquid water morphology and breakthrough in porous layers of polymer electrolyte membrane fuel cell. These suggest presence of large droplets and high liquid saturation at interface of the catalyst layer (CL) and gas diffusion layer (GDL), due to jump in pore size. We develop a model for morphology of liquid phase across multiple porous layers by use of both continuum and breakthrough (percolation) treatments. Using the results of this model we show the liquid morphologies deteriorate the efficiency of electrochemical reactions in CL and increase the water saturation in GDL. Then we show that inserting a microporous layer between CL and GDL reduces both the droplet size and liquid saturation and improves the cell performance.
TL;DR: In this article, the authors presented the dynamic modeling of a single effect two-bed adaption chiller utilizing the composite adsorbent "CaCl 2 confined to KSK silica gel" as an adsorbant and water as adsorbate, which is based on the experimentally confirmed adsorption isotherms and kinetics data.
Abstract: This article presents the dynamic modelling of a single effect two-bed adsorption chiller utilizing the composite adsorbent “CaCl 2 confined to KSK silica gel” as adsorbent and water as adsorbate, which is based on the experimentally confirmed adsorption isotherms and kinetics data. Compared with the experimental data of conventional adsorption chiller based on RD silica gel + water pair, we found that the new working pair provides better cooling capacity and performances. From numerical simulation, it is also found that the cooling capacity can be increased up to 20% of the parent silica gel + water adsorption chiller and the coefficient of performance (COP) can be improved up to 25% at optimum conditions. We also demonstrate here that the best peak chilled water temperature suppression, and the maximum cooling capacity can be achieved by the optimum analysis for both cycles.
TL;DR: In this article, a performance evaluation plot has been proposed, which takes the ratios of heat transfer enhancement and friction factor increase as its two coordinates, where the quadrant of the coordinate where both (Nue/Nu0), (fe/f0) are greater than 1.
Abstract: On the basis of the existing performance evaluation criteria analysis and four assumptions, a performance evaluation plot has been proposed in this paper. This plot takes the ratios of heat transfer enhancement and friction factor increase as its two coordinates. The quadrant of the coordinate where both (Nue/Nu0), (fe/f0) are greater than 1.0 can be divided into four regions. In Region 1 heat transfer is actually deteriorated based on identical pumping power, in Region 2 heat transfer is enhanced based on identical pumping power but deteriorated based on identical pressure drop, in Region 3 heat transfer is enhanced based on identical pressure drop but the increase in friction factor is larger than the enhancement of heat transfer at identical flow rate, and in Region 4 heat transfer enhancement ratio is larger than friction factor increase ratio based on identical flow rate. For some techniques which lead to the reduction of both heat transfer rate and friction factor, the proposed plot is still applicable. Different enhanced techniques for the same reference one can be easily and clearly compared for their effectiveness when enhancement study is based on energy-saving. Five practical examples are provided to show the functions of the plot. 2008 Elsevier Ltd. All rights reserved.
TL;DR: In this paper, the bottom wall of a shallow rectangular micro-channel with carbon nanotubes (CNTs) was tested with a bare copper surface and three separate identical CNT-coated surfaces.
Abstract: Experiments were performed to assess the heat transfer enhancement benefits of coating the bottom wall of a shallow rectangular micro-channel with carbon nanotubes (CNTs). Using water as working fluid, tests were performed with a bare copper surface and three separate, yet identical CNT-coated surfaces. Each of the CNT-coated surfaces was tested repeatedly at the same mass velocity to explore any time dependence of heat transfer performance parameters, especially critical heat flux (CHF). Appreciable differences in the influence of CNT coating were observed at high mass velocities as compared to low. CHF was repeatable at low mass velocities but degraded following repeated tests at high mass velocities, proving high flow velocities cause appreciable changes to the morphology of the CNT-coated surface. SEM images show the initially near-vertical CNTs were bent upon the heated surface at high mass velocities to form a repeated ‘fish-scale’ pattern. Voids between the ‘fish scales’ provided near-zero-angle cavities that enhanced heat transfer in the nucleate boiling region compared to the bare copper surface. While CHF was enhanced by the increased heat transfer area associated with the CNT coating, the enhancement decreased following repeated tests as the CNT fin effect was compromised by the bending.
TL;DR: In this article, a novel photovoltaic/thermal solar-assisted heat pump (PV/T-SAHP) system is described, which is a specially designed directexpansion evaporator, which was laminated with PV cells on the front surface of the thermal absorber, has been adopted in our system to acquire simultaneously thermal energy and electricity from solar radiation.
Abstract: A novel photovoltaic/thermal solar-assisted heat pump (PV/T-SAHP) system is described in this paper. A specially designed direct-expansion evaporator (PV evaporator), which is laminated with PV cells on the front surface of the thermal absorber, has been adopted in our system to acquire simultaneously thermal energy and electricity from solar radiation. A dynamic model of the PV evaporator based on the distributed parameter approach is also presented. Given the instantaneous solar irradiance and ambient temperature, the numerical model is able to output the spatial distributions of refrigerant conditions, including pressure, temperature, vapor quality and enthalpy. A two-dimensional temperature distribution of the evaporator body is also computed. Comparisons between the simulation results and the experimental measurements show that the model is able to give satisfactory predictions. The results show that high electrical and thermal performance can be achieved. The PV efficiency and thermal efficiency are above 12% and 50% during the testing period.
TL;DR: In this paper, 13 prediction methods for flow boiling heat transfer in mini-channels were compared against a new database including 2505 data for 11 liquids covering diameter from 0.21 to 6.05mm.
Abstract: Thirteen prediction methods for flow boiling heat transfer in mini-channels were compared against a new database including 2505 data for 11 liquids covering diameter from 0.21 to 6.05 mm. The results show the Chen method and the Chen-type correlations are not suitable for mini-channels very much; the Lazarek–Black correlation and the Kew–Cornwell correlation are the best two methods. Based on the Lazarek–Black correlation and by introducing Weber number, a modified correlation was proposed.
TL;DR: In this article, experimentally determined pressure drop and heat transfer characteristics of flow of water in a 75-start spirally grooved tube with twisted tape insert are presented, and it is found that the direction of twist (clockwise and anticlockwise) influences the thermohydraulic characteristics.
Abstract: In this paper, experimentally determined pressure drop and heat transfer characteristics of flow of water in a 75-start spirally grooved tube with twisted tape insert are presented. Laminar to fully turbulent ranges of Reynolds numbers have been considered. The grooves are clockwise with respect to the direction of flow. Compared to smooth tube, the heat transfer enhancement due to spiral grooves is further augmented by inserting twisted tapes having twist ratios Y ≃ 10.15, 7.95 and 3.4. It is found that the direction of twist (clockwise and anticlockwise) influences the thermo-hydraulic characteristics. Constant pumping power comparisons with smooth tube characteristics show that in spirally grooved tube with and without twisted tape, heat transfer increases considerably in laminar and moderately in turbulent range of Reynolds numbers. However, for the bare spiral tube and for spiral tube with anticlockwise twisted tape (Y = 10.15), reduction in heat transfer is noticed over a transition range of Reynolds numbers.
TL;DR: In this article, the thermal and fluid-dynamic behavior of a single-pass and double-pass solar parabolic trough collector is analyzed in the presence of recycle at the ends.
Abstract: Detailed numerical simulations of thermal and fluid-dynamic behavior of a single-pass and double-pass solar parabolic trough collector are carried out. The governing equations inside the receiver tube, together with the energy equation in the tube walls and cover wall and the thermal analysis in the solar concentrator were solved iteratively in a segregated manner. The single-pass solar device numerical model has been carefully validated with experimental data obtained by Sandia National Laboratories. The effects of recycle at the ends on the heat transfer are studied numerically shown that the double-pass can enhance the thermal efficiency compared with the single-pass.
TL;DR: In this paper, the thermal and fluid flow characteristics of five heat sinks that have been fabricated by a rapid manufacturing technique known as Selective Laser Melting are presented, and the experimental results for the rectangular fin were compared with data from the literature and were found to be consistent.
Abstract: This study presents the thermal and fluid flow characteristics of five heat sinks that have been fabricated by a rapid manufacturing technique known as Selective Laser Melting. The five heat sinks consist of two conventional designs, the cylindrical pin and rectangular fin array, for comparison purposes, and three novel heat sinks: a staggered elliptical array; a lattice; and a rectangular fin array with rounded corners. The experimental results for the rectangular fin were compared with data from the literature and were found to be consistent. The rectangular fin with rounded corners proved able to transfer the largest amount of heat whilst improving upon the pressure drop performance of the standard rectangular fin array. Although the lattice arrangement made use of the fabrication process’ ability to manufacture heat sinks with high surface area to volume ratios, its performance was limited by the lack of interaction between the cooling air and structure. In terms of both heat transfer performance and pressure drop, the staggered elliptical array, which cannot be manufactured by conventional techniques, outperformed the other heat sinks.
TL;DR: In this paper, the authors proposed a dual-phase model for bioheat equations with blood or tissue temperature as sole unknown temperature by eliminating the tissue or blood temperature from the nonequilibrium model and showed that the phase lag times for heat flux and temperature gradient are very close to each other.
Abstract: Based on a nonequilibrium heat transfer model in the living tissue obtained by performing volume average to the local instantaneous energy equations for blood and tissues, the dual-phase lag bioheat equations with blood or tissue temperature as sole unknown temperature are obtained by eliminating the tissue or blood temperature from the nonequilibrium model. The present dual-phase model successfully overcame the drawbacks of the existing dual-phase lag bioheat equation obtained by simply modifying the classical Pennes bioheat equation. Under the dual-phase model developed in this work, the phase lag times are expressed in terms of the properties of blood and tissue and the interphase convective heat transfer coefficient and blood perfusion rate. The phase lag times for heat flux and temperature gradient for the living tissue are estimated using the available properties from the literature. It is found that the phase lag times for heat flux and temperature gradient for the living tissue are very close to each other.
TL;DR: In this paper, a numerical investigation of the steady magnetohydrodynamics free convection in a rectangular cavity filled with a fluid-saturated porous medium and with internal heat generation has been performed.
Abstract: A numerical investigation of the steady magnetohydrodynamics free convection in a rectangular cavity filled with a fluid-saturated porous medium and with internal heat generation has been performed. A uniform magnetic field, inclined at an angle γ with respect to the horizontal plane, is externally imposed. The values of the governing parameters are the inclined angle γ = 0, π/6, π/4 and π/2, Hartmann number Ha = 0, 1, 5, 10 and 50, Rayleigh number Ra = 10, 100, 10 3 and 10 5 , and the aspect ratio a = 0.01, 0.2, 0.5 and 1 (square cavity). It is shown that the intensity of the core convection is considerably affected by the considered parameters. It is also found that the local Nusselt number Nu Y decreases on the bottom wall as γ increases (magnetic field changes its direction from the horizontal to the vertical direction) and vice versa for the top wall of the cavity. The reported results are in good agreement with the available published work in the literature.
TL;DR: In this paper, the effects of thermal radiation on unsteady boundary layer mixed convection heat transfer problem from a vertical porous stretching surface embedded in porous medium was investigated and the analysis of the results obtained showed that the flow field is influenced appreciably by the unsteadiness parameter, mixed convective parameter, parameter of the porous medium and thermal radiation and suction at wall surface.
Abstract: An analysis is performed to investigate the effects of thermal radiation on unsteady boundary layer mixed convection heat transfer problem from a vertical porous stretching surface embedded in porous medium. The fluid is assumed to be viscous and incompressible. Numerical computations are carried out for different values of the parameters involved in this study and the analysis of the results obtained shows that the flow field is influenced appreciably by the unsteadiness parameter, mixed convection parameter, parameter of the porous medium and thermal radiation and suction at wall surface. With increasing values of the unsteadiness parameter, fluid velocity and temperature are found to decrease in both cases of porous and non-porous media. Fluid velocity decreases due to increasing values of the parameter of the porous medium resulting an increase in the temperature field in steady as well as unsteady case.
TL;DR: In this article, heat transfer coefficients for the nanofluid are presented for Reynolds numbers ranging from 3300 to 13,000 and are compared to the base fluid water on the bases of constant Reynolds number, constant velocity, and constant pumping power.
Abstract: Heat transfer experiments were performed with a water-based nanofluid containing 170-nm silicon carbide particles at a 3.7% volume concentration and having potential commercial viability. Heat transfer coefficients for the nanofluid are presented for Reynolds numbers ranging from 3300 to 13,000 and are compared to the base fluid water on the bases of constant Reynolds number, constant velocity, and constant pumping power. Results were also compared to predictions from standard liquid correlations and a recently altered nanofluid correlation. The slip mechanisms of Brownian diffusion and thermophoresis postulated in the altered correlation were investigated in a series of heating and cooling experiments.
TL;DR: Wen et al. as discussed by the authors showed that the surface conditions are responsible for varying results of pool boiling and suggested that experiments can be fit to the traditional Rohsenow correlation by changing the surface constant, Csf.
Abstract: Nanofluid pool boiling experimental studies have shown mixed results. Recent literature is reviewed and compared here. It is demonstrated here that experiments can be fit to the traditional Rohsenow correlation by changing the surface constant, Csf. Therefore, this study suggests surface conditions are responsible for varying results. Some limited new experimental data are reported for Al2O3/H2O nanofluids using the hot wire method. Relative to the baseline of pure water, boiling incipience occurs 2–3 °C earlier, heat transfer is enhanced 25–40%, but sub-cooled boiling deteriorates. These results are essentially in agreement with most earlier studies showing enhancement [S.K. Das, N. Putra, W. Roetzel, Pool boiling of nanofluids on horizontal narrow tubes, Int. J. Multiphase Flow 29 (8) (2003) 1237–1247; D. Wen, Y. Ding, Experimental investigation into the pool boiling heat transfer of aqueous based c-alumina nanofluids, J. Nanoparticle Res. 7 (2005) 265–274; S. Witharana, Boiling of refrigerants on enhanced surfaces and boiling of nanofluids, Ph.D. thesis, The Royal Institute of Technology, 2003; S.M. You, J.H. Kim, K.H. Kim, Effect of nanoparticles on critical heat flux of water in pool boiling heat transfer, Appl. Phys. Lett. 83 (2003) 3374–3376].
TL;DR: In this paper, the influence of heat convection and internal heat generation on structural designs is investigated. But the authors focus on the influence on the heat transfer coefficient and heat conduction on the structure surface.
Abstract: In structural designs considering thermal loading, in addition to heat conduction within the structure, the heat convection upon the structure’s surface can significantly influence optimal design configurations. In this paper, we focus on the influence of design-dependent effects upon heat convection and internal heat generation for optimal designs developed using a topology optimization scheme. The method for extracting the structural boundaries for heat convection loads is constructed using a Hat function, and heat convection shape dependencies are taken into account in the heat transfer coefficient using a surrogate model. Several numerical examples are presented to confirm the usefulness of the proposed method.