Showing papers in "International Journal of Heat and Mass Transfer in 2008"

Effective viscosities and thermal conductivities of aqueous nanofluids containing low volume concentrations of Al2O3 nanoparticles

Abstract: Aqueous nanofluids containing low volume concentrations of Al2O3 nanoparticles in the 0.01–0.3 vol.% range were produced and characterized. Measurements of zeta potential and TEM micrograph of the alumina nanoparticles in the Al2O3–water nanofluids show that the alumina nanoparticles can be best dispersed and stabilized in DI water with little evidence of aggregation at 5 h of ultrasonic vibration. Viscosity measurements show that the viscosity of the Al2O3–water nanofluids significantly decreases with increasing temperature. Furthermore, the measured viscosities of the Al2O3–water nanofluids show a nonlinear relation with the concentration even in the low volume concentration (0.01%–0.3%) range, while the Einstein viscosity model clearly predicts a linear relation, and exceed the Einstein model predictions. In contrast to viscosity, the measured thermal conductivities of the dilute Al2O3–water nanofluids increase nearly linearly with the concentration, agree well with the predicted values by the Jang and Choi model, and are consistent in their overall trend with previous data at higher concentrations.

Numerical simulation of natural convection of nanofluid in a square enclosure: Effects due to uncertainties of viscosity and thermal conductivity

Abstract: The present study aims to identify effects due to uncertainties in effective dynamic viscosity and thermal conductivity of nanofluid on laminar natural convection heat transfer in a square enclosure. Numerical simulations have been undertaken incorporating a homogeneous solid–liquid mixture formulation for the two-dimensional buoyancy-driven convection in the enclosure filled with alumina–water nanofluid. Two different formulas from the literature are each considered for the effective viscosity and thermal conductivity of the nanofluid. Simulations have been carried out for the pertinent parameters in the following ranges: the Rayleigh number, Raf = 103–106 and the volumetric fraction of alumina nanoparticles, ϕ = 0–4%. Significant difference in the effective dynamic viscosity enhancement of the nanofluid calculated from the two adopted formulas, other than that in the thermal conductivity enhancement, was found to play as a major factor, thereby leading to contradictory results concerning the heat transfer efficacy of using nanofluid in the enclosure.

Effect of aggregation and interfacial thermal resistance on thermal conductivity of nanocomposites and colloidal nanofluids

Abstract: We analyzed the role of aggregation and interfacial thermal resistance on the effective thermal conductivity of nanofluids and nanocomposites. We found that the thermal conductivity of nanofluids and nanocomposites can be significantly enhanced by the aggregation of nanoparticles into clusters. The value of the thermal conductivity enhancement is determined by the cluster morphology, filler conductivity and interfacial thermal resistance. We also compared thermal conductivity enhancement due to aggregation with that associated with high-aspect ratio fillers, including fibers and plates.

A Review of two-phase flow dynamic instabilities in tube boiling systems

Abstract: The earliest research in the field of two-phase flow was conducted by Lorentz (1909) The studies on the analysis of two-phase flow instabilities by Ledinegg (1938) created considerable interest concerning the phenomenon of thermally induced flow instability in two-phase flow systems The objective of this review is to sum up the experimental and theoretical work carried out by various investigators over a period of several years, demonstrating and explaining three main instability modes of two-phase flow dynamic instabilities, namely, density-wave type, pressure-drop type and thermal oscillations, encountered in various boiling flow channel systems The typical experimental investigations of these instabilities in tube boiling systems are indicated and the most popular models to predict the two-phase flow dynamic instabilities, namely the homogenous flow model and the drift-flux models are clarified with the solution examples and the validation of the model results with experimental findings are also provided

Thermal instability of nanofluids in natural convection

Abstract: Thermal (Benard) instability in nanofluids is investigated in this work. Emphasizing the combined behaviors of Brownian motion and thermophoresis of nanoparticles, the critical Rayleigh number is shown to be lower by one to two orders of magnitude than that for regular fluids. The highly promoted turbulence increases the energy bearing capacity of nanofluids, which could result in higher overall heat transfer coefficient than the increase of the effective thermal conductivity alone. The dominating groups are extracted from the nondimensional analysis. Close form solutions for the Rayleigh number are derived from the method of eigenfunction expansions and the weighted residual method.

A numerical study of natural convection in a square enclosure with a circular cylinder at different vertical locations

Abstract: Numerical calculations are carried out for natural convection induced by a temperature difference between a cold outer square enclosure and a hot inner circular cylinder. A two-dimensional solution for unsteady natural convection is obtained, using the immersed boundary method (IBM) to model an inner circular cylinder based on the finite volume method for different Rayleigh numbers varying over the range of 103–106. The study goes further to investigate the effect of the inner cylinder location on the heat transfer and fluid flow. The location of the inner circular cylinder is changed vertically along the center-line of square enclosure. The number, size and formation of the cell strongly depend on the Rayleigh number and the position of the inner circular cylinder. The changes in heat transfer quantities have also been presented.

Effects of inlet/outlet configurations on flow boiling instability in parallel microchannels

Abstract: A simultaneous visualization and measurement study has been carried out to investigate effects of inlet/outlet configurations on flow boiling instabilities in parallel microchannels, having a length of 30 mm and a hydraulic diameter of 186 μm. Three types of inlet/outlet configurations were investigated. Fluid flow entering to and exiting from the microchannels with the Type-A connection was restricted because the inlet and outlet conduits were perpendicular to the microchannels. The fluid flow had no restriction in entering to and existing from the microchannels with the Type-B connection. In the Type-C connection, fluid flow was restricted in entering each microchannel but was not restricted in exiting from the microchannels. It is found that amplitudes of temperature and pressure oscillations in the Type-B connection are much smaller than those in the Type-A connection under the same heat flux and mass flux conditions. On the other hand, nearly steady flow boiling exists in the parallel microchannels with the Type-C connection under the experimental conditions. Therefore, this configuration is recommended for high-heat-flux microchannel applications. As predicted, the stability threshold is determined by the minimum in the pressure-drop-versus-flow-rate curve. The pressure drop and heat transfer coefficient versus vapor quality for flow boiling in microchannels with the Type-C connection are presented. It is found that experimental data of pressure drop are higher and heat transfer coefficients are lower for boiling flow at high vapor quality in microchannels than those predicted from correlation equations for boiling flow in macrochannels, due to local dryout.

A synthesis of fluid and thermal transport models for metal foam heat exchangers

Abstract: Metal foam heat exchangers have considerable advantages in thermal management and heat recovery over several commercially available heat exchangers. In this work, the effects of micro structural metal foam properties, such as porosity, pore and fiber diameters, tortuosity, pore density, and relative density, on the heat exchanger performance are discussed. The pertinent correlations in the literature for flow and thermal transport in metal foam heat exchangers are categorized and investigated. Three main categories are synthesized. In the first category, the correlations for pressure drop and heat transfer coefficient based on the microstructural properties of the metal foam are given. In the second category, the correlations are specialized for metal foam tube heat exchangers. In the third category, correlations are specialized for metal foam channel heat exchangers. To investigate the performance of the foam filled heat exchangers in comparison with the plain ones, the required pumping power to overcome the pressure drop and heat transfer rate of foam filled and plain heat exchangers are studied and compared. A performance factor is introduced which includes the effects of both heat transfer rate and pressure drop after inclusion of the metal foam. The results indicate that the performance will be improved substantially when a metal foam is inserted in the tube/channel.

Non-Fourier analysis of skin biothermomechanics

Abstract: Biothermomechanics of skin is highly interdisciplinary, involving bioheat transfer, burn damage, biomechanics and physiology. Comprehension of the phenomena of heat transfer and related thermomechanics in skin tissue is of great importance and can contribute to a variety of medical applications. Due to the “lengthy” thermal relaxation time in biological tissue, non-Fourier thermal behaviour has been experimentally observed, attracting increasingly more attention to this phenomenon. The aim of this study is to review previous researches on the non-Fourier heat transfer process and to develop a computational approach to examine this non-Fourier process and its influence on the mechanical response in skin tissue. The dual-phase-lag (DPL) model is first used to model bioheat transfer across the tissue. Together with the thermal wave model, the non-Fourier thermomechanical behaviour of the tissue is analyzed under various surface heating boundary conditions. For single-layer tissue model, exact solutions for temperature, thermal stress and thermal damage fields are derived; for multi-layer structural models, numerical solutions are obtained with the finite difference method. Large discrepancies are found to exist amongst the predictions of Pennes model, thermal wave model and dual-phase-lag model, while different DPL bioheat transfer models give similar predictions.

Experimental study and theoretical analysis of local heat transfer distribution between smooth flat surface and impinging air jet from a circular straight pipe nozzle

Abstract: An experimental investigation is performed to study the effect of jet-to-plate spacing and Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 12,000 and 28,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Measurements for the static wall pressure distribution due to impinging jet at different jet-to-plate spacing are made. The local heat transfer distributions are analyzed based on theoretical predictions and experimental results of the fluid flow characteristics in the various regions of jet impingement. The heat transfer at the stagnation point is analyzed from the static wall pressure distribution. Semi-analytical solution for heat transfer in the stagnation region is obtained assuming an axisymmetric laminar boundary layer with favourable pressure gradient. The heat transfer in the wall jet region is studied considering fluid flow over a flat plate of constant heat flux. However, heat transfers in the transition region are explained from reported fluid dynamic behaviour in this region. Correlations for the local Nusselt numbers in different regions are obtained and compared with experimental results.

Numerical and experimental investigation of two phase flow during boiling in a coiled tube

Abstract: A numerical simulation, using the VOF multiphase flow model, and the corresponding experiments were conducted to investigate the boiling flow of R141B in a horizontal coiled tube. The numerical predictions of phase evolution were in a good agreement with the experimental observations, and the two phase flow in the tube bends was much more complicated due to the influence of liquid–vapor interaction with the interface evolution. The associated heat transfer was also considered. It was found that the temperature profile in the two phase flow was significantly affected by the phase distribution and higher temperature always appears in the vapor region.

Modeling and prediction of the effective thermal conductivity of random open-cell porous foams

Abstract: Although highly desirable, accurate prediction of the effective thermal conductivity of high-porosity open-cell porous foam materials has remained to be a challenging problem. Aiming at this thorny obstacle, we have developed a random generation-growth method to reproduce the microstructures of open-cell foam materials via computer modeling, and then solve the energy transport equations through the complex structure by using a high-efficiency lattice Boltzmann method in this contribution. The effective thermal conductivities of open-cell foam materials are thus numerically calculated and the predictions are compared with the existing experimental data. Since the porosity is high, the predicted thermal conductivity caused by thermal conduction is lower than the measured data when the thermal conductivity of either component is very low and the radiation heat transfer is non-negligible. After considering the radiation effect, the numerical predictions agree rather well with the experimental data. The radiation influence is diminishing as the material porosity decreases. In general the effective thermal conductivity of open-cell foam materials is much higher than that of granular materials of the same components due to the enhanced heat transfer by the inner netlike morphology of the foam materials.

Bio-heat transfer analysis during short pulse laser irradiation of tissues

Abstract: The objective of this paper is to analyze the temperature distributions and heat affected zone in skin tissue medium when irradiated with either a collimated or a focused laser beam from a short pulse laser source. Experiments are performed on multi-layer tissue phantoms simulating skin tissue with embedded inhomogeneities simulating subsurface tumors and as well as on freshly excised mouse skin tissue samples. Two types of lasers have been used in this study – namely a Q-switched pulsed 1064 nm Nd:YAG short pulse laser having a pulse width of 200 ns and a 1552 nm diode short pulsed laser having a pulse width of 1.3 ps. Experimental measurements of axial and radial temperature distribution in the tissue medium are compared with the numerical modeling results. For numerical modeling, the transient radiative transport equation is first solved using a discrete ordinates method for obtaining the intensity distribution and radiative heat flux inside the tissue medium. Then the temperature distribution is obtained by coupling the bio-heat transfer equation with either hyperbolic non-Fourier or parabolic Fourier heat conduction model. The hyperbolic heat conduction equation is solved using MacCormack’s scheme with error terms correction. It is observed that experimentally measured temperature distribution is in good agreement with that predicted by hyperbolic heat conduction model. The experimental measurements demonstrate that converging laser beam focused directly at the subsurface location can produce desired high temperature at that location compared to that produced by collimated laser beam for the same laser parameters. Finally the ablated tissue removal is characterized using histological studies as a function of laser parameters.

A general bioheat transfer model based on the theory of porous media

Abstract: A volume averaging theory (VAT) established in the field of fluid-saturated porous media has been successfully exploited to derive a general set of bioheat transfer equations for blood flows and its surrounding biological tissue. A closed set of macroscopic governing equations for both velocity and temperature fields in intra- and extravascular phases has been established, for the first time, using the theory of anisotropic porous media. Firstly, two individual macroscopic energy equations are derived for the blood flow and its surrounding tissue under the thermal non-equilibrium condition. The blood perfusion term is identified and modeled in consideration of the transvascular flow in the extravascular region, while the dispersion and interfacial heat transfer terms are modeled according to conventional porous media treatments. It is shown that the resulting two-energy equation model reduces to Pennes model, Wulff model and their modifications, under appropriate conditions. Subsequently, the two-energy equation model has been extended to the three-energy equation version, in order to account for the countercurrent heat transfer between closely spaced arteries and veins in the circulatory system and its effect on the peripheral heat transfer. This general form of three-energy equation model naturally reduces to the energy equations for the tissue, proposed by Chato, Keller and Seiler. Controversial issues on blood perfusion, dispersion and interfacial heat transfer coefficient are discussed in a rigorous mathematical manner.

Saturated flow boiling heat transfer and pressure drop in silicon microchannel arrays

Abstract: Flow boiling in arrays of parallel microchannels is investigated using a silicon test piece with imbedded discrete heat sources and integrated local temperature sensors. The microchannels considered range in width from 102 μm to 997 μm, with the channel depth being nominally 400 μm in each case. Each test piece has a footprint of 1.27 cm by 1.27 cm with parallel microchannels diced into one surface. Twenty five microsensors integrated into the microchannel heat sinks allow for accurate local temperature measurements over the entire test piece. The experiments are conducted with deionized water which enters the channels in a purely liquid state. Results are presented in terms of temperatures and pressure drop as a function of imposed heat flux. The experimental results allow a critical assessment of the applicability of existing models and correlations in predicting the heat transfer rates and pressure drops in microchannel arrays, and lead to the development of models for predicting the two-phase pressure drop and saturated boiling heat transfer coefficient.

Microfluidics of nano-drug delivery

Abstract: After a brief review of microfluidics, a bio-MEMS application in terms of nanofluid flow in microchannels is presented. Specifically, the transient 3-D problem of controlled nano-drug delivery in a heated microchannel has been numerically solved to gain new physical insight and to determine suitable geometric and operational system parameters. Computer model accuracy was verified via numerical tests and comparisons with benchmark experimental data sets. The overall design goals of near-uniform nano-drug concentration at the microchannel exit plane and desired mixture fluid temperature were achieved with computer experiments considering different microchannel lengths, nanoparticle diameters, channel flow rates, wall heat flux areas, and nanofluid supply rates. Such micro-systems, featuring controlled transport processes for optimal nano-drug delivery, are important in laboratory-testing of predecessors of implantable smart devices as well as for analyzing pharmaceuticals and performing biomedical precision tasks.

Microchannel size effects on local flow boiling heat transfer to a dielectric fluid

Abstract: Heat transfer with liquid–vapor phase change in microchannels can support very high heat fluxes for use in applications such as the thermal management of high-performance electronics. However, the effects of channel cross-sectional dimensions on the two-phase heat transfer coefficient and pressure drop have not been investigated extensively. In the present work, experiments are conducted to investigate the local flow boiling heat transfer of a dielectric fluid, Fluorinert FC-77, in microchannel heat sinks. Experiments are performed for mass fluxes ranging from 250 to 1600 kg/m2 s. Seven different test pieces made from silicon and consisting of parallel microchannels with nominal widths ranging from 100 to 5850 μm, all with a nominal depth of 400 μm, are considered. An array of temperature sensors on the substrate allows for resolution of local temperatures and heat transfer coefficients. The results of this study show that for microchannels of width 400 μm and greater, the heat transfer coefficients corresponding to a fixed wall heat flux as well as the boiling curves are independent of channel size. Also, heat transfer coefficients and boiling curves are independent of mass flux in the nucleate boiling region for a fixed channel size, but are affected by mass flux as convective boiling dominates. A strong dependence of pressure drop on both channel size and mass flux is observed. The experimental results are compared to predictions from a number of existing correlations for both pool boiling and flow boiling heat transfer.

A note on stability in three-phase-lag heat conduction

Abstract: In this note we consider two cases in the theory of the heat conduction models with three-phase-lag. For each one we propose a suitable Lyapunov function. These functions are relevant tools which allow to study several qualitative properties. We obtain conditions on the material parameters to guarantee the exponential stability of solutions. The spectral analysis complements the results and we show that if the conditions obtained to prove the exponential stability are not satisfied, then we can obtain the instability of solutions for suitable domains. We believe that this kind of results is fundamental to clarify the applicability of the models.

A new structural model of effective thermal conductivity for heterogeneous materials with co-continuous phases

Abstract: A new structural model for a heterogeneous material with multiple continuous phases is proposed. The corresponding equation for effective thermal conductivity was derived using three methods. The new model is substantially different from the conventional five fundamental structural models (Series, Parallel, two forms of Maxwell–Eucken, Effective Medium Theory). The model has two applications. First, as a new fundamental structural model to produce composite models using the combinatory rules previously proposed by J.F. Wang, J.K. Carson, M.F. North, D. J. Cleland, A new approach to modelling the effective thermal conductivity of heterogeneous materials, International Journal of Heat and Mass Transfer, 49 (17–18) (2006) 3075–3083. Second, to narrow the bounds of the effective thermal conductivity for heterogeneous materials where the physical structure can be characterised into general classes.

Boundary layer flow over a shrinking sheet with power-law velocity

Abstract: In this work, the boundary layers over a continuously shrinking sheet with a power-law surface velocity and mass transfer were investigated. Based on the boundary layer assumptions, the similarity equations with a controlling parameter β were obtained and solved numerically. Theoretical analysis was conducted for certain special conditions and exact solutions were derived for β = −1 and β = −2 and also for the power index m = - 1 . Numerical techniques were used to solve the similarity equation for other parameters. Quite different and interesting solution behaviors were found for a shrinking sheet compared with a stretching sheet. Multiple solutions were obtained for certain mass transfer parameter and controlling parameter β. Velocity overshoot near the wall and near the boundary layer edge were observed for certain solution branches. The current results for a power-law shrinking sheet offer quite interesting nonlinear behaviors and greatly enrich the solution and understanding of boundary layers.

Numerical study on laminar convection heat transfer in a rectangular channel with longitudinal vortex generator. Part A: Verification of field synergy principle

Abstract: This study presents numerical computation results on laminar convection heat transfer in a rectangular channel with a pair of rectangular winglets longitudinal vortex generator punched out from the lower wall of the channel. The effect of the punched holes and the thickness of the rectangular winglet pair to the fluid flow and heat transfer are numerically studied. It is found that the case with punched holes has more heat transfer enhancement in the region near to the vortex generator and lower average flow frictional coefficient compared with the case without punched holes. The thickness of rectangular winglet can cause less heat transfer enhancement in the region near to the vortex generator and almost has no significant effect on the total pressure drop of the channel. The effects of Reynolds number (from 800 to 3000), the attack angle of vortex generator (15°, 30°, 45°, 60° and 90°) were examined. The numerical results were analyzed from the viewpoint of field synergy principle. It was found that the essence of heat transfer enhancement by longitudinal vortex can be explained very well by the field synergy principle, i.e., when the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient, the heat transfer in the present channels will be enhanced. Longitudinal vortices (LVs) improve the synergy between velocity and temperature field not only in the region near LVG but also in the large downstream region of longitudinal vortex generator. So LVs enable to enhance the global heat transfer of channel. Transverse vortices (TVs) only improve the synergy in the region near VG. So TVs can only enhance the local heat transfer of channel.

Numerical investigation of a PCM-based heat sink with internal fins: Constant heat flux

Abstract: Melting of a phase change material (PCM) is studied in a heat sink with vertical internal fins and a horizontal base to which a constant heat flux is applied. The phase change material is stored between the fins. A detailed parametric investigation explores various fin height and thickness, PCM layer thickness, and applied heat flux. Transient numerical simulations are performed using the Fluent 6 software. The results show how the transient phase change process depends on the heat flux from the base, heat capacity of the PCM, and fin dimensions. Dimensional analysis of the results is performed, and the generalized results are presented in terms of the melt fractions and Nusselt numbers vs. the Fourier, Stefan and Rayleigh numbers.

New prediction methods for CO2 evaporation inside tubes: Part I – A two-phase flow pattern map and a flow pattern based phenomenological model for two-phase flow frictional pressure drops

Abstract: Reference LTCM-ARTICLE-2008-035doi:10.1016/j.ijheatmasstransfer.2007.04.002View record in Web of Science Record created on 2008-01-11, modified on 2017-05-10

Numerical simulation of heat transfer and fluid flow past a rotating isothermal cylinder – A LBM approach

Abstract: In this paper, a viscous fluid flowing past a rotating isothermal cylinder with heat transfer is studied and simulated numerically by the lattice Boltzmann method (LBM). A numerical strategy for dealing with curved and moving boundaries of second-order accuracy for both velocity and temperature fields is proposed and presented. The numerical strategy and method are validated by comparing the present numerical results of flow without heat transfer with those of available previous theoretical, experimental and numerical studies, showing good agreements. On this basis, the convective heat transfer performance in such rotational boundary environments is further studied and validated; the numerical results are reported in the first time. The effects of the peripheral-to-translating-speed ratio, Reynolds number and Prandtl number on flow and heat transfer are discussed in details.

High heat flux flow boiling in silicon multi-microchannels – Part II: Heat transfer characteristics of refrigerant R245fa

Abstract: Reference EPFL-ARTICLE-147386doi:10.1016/j.ijheatmasstransfer.2008.03.007View record in Web of Science Record created on 2010-03-19, modified on 2017-05-10

Assessment of performance of turbulence models in predicting supercritical pressure heat transfer in a vertical tube

Abstract: Computational simulations of heat transfer to fluids at a supercritical pressure have been performed using an ‘in-house’ CFD code written for two-dimensional axisymmetric flow and heat transfer based on the Favre averaging approach. Results are compared with recently available direct numerical simulations (DNS) which provide a benchmark dataset ideal for model assessment. The objective of the present study is to evaluate the performance of low-Reynolds number turbulence models in predicting mixed convection heat transfer to fluids at supercritical pressure, especially paying attention to the features which enable them to respond to the modifications of the turbulence field due to influences of flow acceleration and buoyancy. It has been found that a group of turbulence models which were previously found closely reproducing mixed convection under conditions of constant properties do not perform well for flows considered in the present study due to an over-response to changes in the flow. Models which were less successful previously perform better. The V2F model performs the best among all models tested. For strong-buoyancy-influenced cases, most models are able to reproduce turbulence recovery reasonably well but not the improvement on heat transfer. This is attributed, at least partly, to the inability of turbulence models in reproducing turbulent heat flux using a constant turbulent Prandtl number. The influence of the lack of a suitable description of the axial turbulent heat flux has been shown to be insignificant except immediately after the commencement of heating.

Frost formation on a super-hydrophobic surface under natural convection conditions

Abstract: In this paper, the frost deposition phenomena on a cold super-hydrophobic surface whose contact angle with water is 162° were observed of the formation of water droplets, the freezing process, the formation of initial frost crystals and the frost layer structure. The frost layer structure formed on the super-hydrophobic surface shows remarkable differences to that on a plain copper surface: the structure is weaker, looser, thin and easy to be removed and most importantly, it is of a very special pattern, a pattern similar to a cluster of chrysanthemum petals, a frost layer structure that has not been reported before. The experimental results also show that the surface has a strong ability to restrain frost growth. The frost deposition on the surface was delayed for 55 min compared with the plain copper surface under the tested conditions.

A mathematical model for analyzing the thermal characteristics of a flat micro heat pipe with a grooved wick

Abstract: A mathematical model is developed for predicting the thermal performance of a flat micro heat pipe with a rectangular grooved wick structure. The effects of the liquid–vapor interfacial shear stress, the contact angle, and the amount of liquid charge are accounted for in the present model. In particular, the axial variations of the wall temperature and the evaporation and condensation rates are considered by solving the one-dimensional conduction equation for the wall and the augmented Young–Laplace equation, respectively. The results obtained from the proposed model are in close agreement with several existing experimental data in terms of the wall temperatures and the maximum heat transport rate. From the validated model, it is found that the assumptions employed in previous studies may lead to significant errors for predicting the thermal performance of the heat pipe. Finally, the maximum heat transport rate of a micro heat pipe with a grooved wick structure is optimized with respect to the width and the height of the groove by using the proposed model. The maximum heat transport rate for the optimum conditions is enhanced by approximately 20% compared to existing experimental results.

Spray water cooling heat transfer at high temperatures and liquid mass fluxes

Abstract: Spray water cooling is an important technology used in industry for the cooling of materials from temperatures up to 1800 K. The heat transfer coefficient in the so-called steady film boiling regime is known to be a function of the water impact density. Below a specific surface temperature TL, the heat transfer coefficient shows a strong dependence on temperature (Leidenfrost effect). These findings are the results of complex self-organizing two-phase boiling heat transfer phenomena. The heat transfer coefficient was measured by an automated cooling test stand (instationary method) under clean (non-oxidizing) surface conditions. Compared to the common thought, an additional temperature dependency in the high temperature regime was found. The heat transfer from the material to the outflowing spray water is explained by a simple model of the two-phase flow region. From the experimental data, an analytic correlation for the dependence of the heat transfer coefficient α as an analytic function of water impact density VS and temperature ΔT is provided. For water temperatures around 291 K, surface temperatures between 473 and 1373 K, i.e. ΔT > 180 K and water impact densities between VS = 3 and 30 kg/(m2 s) the heat transfer coefficient α was measured. The spray was produced with full cone nozzles (vd ≈ 13–15 m/s, dd ≈ 300–400 μm).

A new methodology of studying the dynamics of water sorption/desorption under real operating conditions of adsorption heat pumps: Experiment

Abstract: In this paper we proposed and tested a new methodology of studying the kinetics of water vapour sorption/desorption under operating conditions typical for isobaric stages of sorption heat pumps. The measurements have been carried out on pellets of composite sorbent SWS-1L (CaCl 2 in silica KSK) placed on a metal plate. Temperature of the plate was changed as it takes place in real sorption heat pumps, while the vapour pressure over the sorbent was maintained almost constant (saturation pressures corresponding to the evaporator temperature of 5 °C and 10 °C and the condenser temperature of 30 °C and 35 °C). Near-exponential behaviour of water uptake on time was found for most of the experimental runs. Characteristic time τ of isobaric adsorption (desorption) was measured for one layer of loose grains having a size between 1.4 mm and 1.6 mm for different heating/cooling scenarios and boundary conditions of an adsorption heat pump. Maximum specific power estimated from the τ -values can exceed 1.0 kW/kg of dry adsorbent, that gives proof to the idea of compact adsorption units for energy transformation with loose SWS grains.