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Showing papers in "Heat Transfer Engineering in 2004"


Journal ArticleDOI
TL;DR: In this paper, a flow boiling correlation for large diameter tubes developed by Kandlikar [1, 2] is modified for flow boiling in minichannels by using the laminar single-phase heat transfer coefficient for all liquid flow.
Abstract: Flow boiling in mini- and microchannels offer very high heat transfer capabilities and find applications in many emerging technologies, such as electronics cooling and fuel cells. The low flow rate employed in such geometries, coupled with the small flow channels, often results in a laminar flow with all flow as liquid. Since the single-phase flow with all liquid is in the laminar range, the flow boiling correlations developed for conventional tubes with an inner diameter larger than 3 mm and turbulent flow need to be carefully reviewed. In the present work, flow boiling correlation for large diameter tubes developed by Kandlikar [1, 2] is modified for flow boiling in minichannels by using the laminar single-phase heat transfer coefficient for all liquid flow. The correlation is also extended for flow boiling in microchannels using the nucleate boiling as the dominant part of the original correlation. The trends in heat transfer coefficient versus quality are compared in the laminar and deep laminar regio...

308 citations


Journal ArticleDOI
TL;DR: In this article, an analytical slip-flow model based on second-order boundary conditions was proposed for gaseous flow in rectangular microchannels, which is valid for Knudsen numbers up to about 0.25, whereas the first-order model no longer accurate for values higher than 0.05.
Abstract: An analytical slip-flow model based on second-order boundary conditions was proposed for gaseous flow in rectangular microchannels. An experimental setup has been designed for the measurement of gaseous micro flow rates under controlled temperature and pressure conditions. Data relative to nitrogen and helium flows through rectangular microchannels, from 4.5 to 0.5 μm in depth and with aspect ratios from 1–9%, are presented and analyzed. A method is proposed to eliminate the main source of uncertainty, which is the imprecision when measuring the dimensions of the microchannel cross-section. It is shown that in rectangular microchannels, the proposed second-order model is valid for Knudsen numbers up to about 0.25, whereas the first-order model is no longer accurate for values higher than 0.05. The best fit is found for a tangential momentum accommodation coefficient σ = 0.93, both with helium and nitrogen.

250 citations


Journal ArticleDOI
TL;DR: In this article, a roadmap for single-phase cooling technology is presented to identify research opportunities in meeting the cooling demands of future IC chips using three-dimensional microchannels that incorporate either microstructures in the channel or grooves in channel surfaces.
Abstract: The increased circuit density on today's computer chips is reaching the heat dissipation limits for air-cooling technology. The direct liquid cooling of chips is being considered as a viable alternative. This paper reviews liquid cooling with internal flow channels in terms of technological options and challenges. The possibilities presented herein indicate a four- to ten-fold increase in heat flux over the air-cooled systems. The roadmap for single-phase cooling technology is presented to identify research opportunities in meeting the cooling demands of future IC chips. The use of three-dimensional microchannels that incorporate either microstructures in the channel or grooves in the channel surfaces may lead to significant enhancements in single-phase cooling. A simplified and well-established fabrication process is described to fabricate both classes of three-dimensional microchannels. Proof-of-concept microchannels are presented to demonstrate the efficacy of the fabrication process in fabricating com...

185 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the cooling performance of piezoelectric fans and found that they offer significant localized cooling, exceeding enhancements in convective heat transfer coefficients of 100%, while exhibiting low power consumption, minimal noise, and small dimensions.
Abstract: Piezoelectric fans are investigated as a cooling technology for the thermal management of electronic devices. Flow visualization experiments are conducted to better understand the physics of fan operation. Prototypes of the fans are built and tested to assess their feasibility and cooling performance and determine optimal locations for the fans. An enclosure the size of a cellular phone and a commercially available laptop computer are used to demonstrate the cooling feasibility of the fans. Piezoelectric fans are found to offer significant localized cooling, exceeding enhancements in convective heat transfer coefficients of 100%, while exhibiting low power consumption, minimal noise, and small dimensions. Performance metrics for piezoelectric fans should be based on heat transfer characteristics, such as the percent increase in the heat transfer coefficient of the system. Optimization techniques that maximize the electromechanical coupling factor (EMCF) can be used to design efficient fans.

154 citations


Journal ArticleDOI
TL;DR: In this article, the authors present an overview of the use of flow visualization in micro-and mini-channel geometries for the development of pressure drop and heat transfer models during condensation of refrigerants.
Abstract: This paper presents an overview of the use of flow visualization in micro- and mini-channel geometries for the development of pressure drop and heat transfer models during condensation of refrigerants. Condensation flow mechanisms for round, square, and rectangular tubes with hydraulic diameters in the range of 1–5 mm for 0 < x < 1, and 150 kg/m2-s and 750 kg/m2-s were recorded using unique experimental techniques that permit flow visualization during the condensation process. The effect of channel shape and miniaturization on the flow regime transitions was documented. The flow mechanisms were categorized into four different flow regimes: intermittent flow, wavy flow, annular flow, and dispersed flow. These flow regimes were further subdivided into several flow patterns within each regime. It was observed that the intermittent and annular flow regimes become larger as the tube hydraulic diameter is decreased, and at the expense of the wavy flow regime. These maps and transition lines can be used to predi...

123 citations


Journal ArticleDOI
TL;DR: In this paper, a wide-ranging research program was conducted on microchannel heat sinks and micropumps to understand fluid flow and heat transfer in microchannels and to identify pumping requirements and suitable mechanisms for pumping in micro-channels.
Abstract: Microchannel heat sinks are widely regarded as being among the most effective heat removal techniques from space-constrained electronic devices. However, the fluid flow and heat transfer in microchannels is not fully understood. The pumping requirements for flow through microchannels are also very high, and none of the micropumps in the literature is truly suitable for this application. Results are reported from a wide-ranging research program being conducted on microchannel heat sinks and micropumps to understand fluid flow and heat transfer in microchannels and to identify pumping requirements and suitable mechanisms for pumping in microchannels. In particular, experiments have been performed to show that conventional correlations for fluid flow and heat transfer adequately predict the behavior in microchannels of hydraulic diameters as small as 250 μ m. Pumping requirements of microchannel heat sinks have been analyzed, and the size of the microchannels have been optimized for minimum pumping requireme...

106 citations


Journal ArticleDOI
Sung Jin Kim1
TL;DR: In this article, the authors presented three optimization methods by which the thermal resistance of the microchannel heat sink can be minimized: the fin model, the porous medium model, and the numerical optimization method.
Abstract: Since the pioneering introduction of the microchannel heat sink by Tuckerman and Pease [1], many investigators have studied heat transfer phenomena within these heat sinks. Their efforts have been aimed at presenting an optimized structure for the microchannel heat sink. This paper presents three optimization methods by which the thermal resistance of the microchannel heat sink can be minimized: the fin model, the porous medium model, and the numerical optimization method. Assumptions used in the fin model are shown to be invalid for large values of the aspect ratio, while the porous medium model is shown to accurately predict the thermal performance of the microchannel heat sink. Because of this defect, the fin model fails to provide the design variables. In addition, the optimized design variables and the corresponding thermal resistances are presented using the optimization methods based on the porous medium model and the numerical simulation under the constraint of a maximum pumping power.

101 citations


Journal ArticleDOI
Roger R. Schmidt1
TL;DR: A microprocessor liquid cooled minichannel heat sink will be described and its performance presented as it applies to a current microprocessor (IBM Power4) chip.
Abstract: The volumetric heat dissipated by computer equipment at each level of the package from the chip to the chassis is having a tremendous impact on the thermal management of computer equipment. Because of the consumer's insatiable desire for increased performance, the competitive pressures are driving the computer manufacturer to pack as much processor/memory performance within the smallest volume possible. The consumer views high performance in a compact package as a benefit. These market pressures seem to be in direct conflict with the desire to continue to provide air cooling solutions for the foreseeable future. Because of these trends in power and package design, other cooling technologies beside air are now becoming viable, techniques, but each must be weighed with many other factors that influence the cooling technology selected. These factors will be discussed along with two specific IBM server packages and their associated cooling technology employed. Finally a microprocessor liquid cooled minichanne...

72 citations


Journal ArticleDOI
TL;DR: In this article, a scaled down model study is performed using water as the model fluid in a rectangular glass tank heated from the sides, and the convective flow and the resulting thermal stratification phenomenon in the rectangular tank are studied through flow visualization, temperature measurement and corresponding numerical simulations.
Abstract: A study of convective heat transfer in a cryogenic storage vessel is carried out numerically and experimentally. A scaled down model study is performed using water as the model fluid in a rectangular glass tank heated from the sides. The convective flow and the resulting thermal stratification phenomenon in the rectangular tank are studied through flow visualization, temperature measurement, and corresponding numerical simulations. It is found that a vortex-like flow near the top surface leads to a well-mixed region there, below which the fluid is thermally stratified. In addition, in an attempt to simulate the actual conditions, a numerical study is performed on a cylindrical cavity filled with liquid hydrogen $(LH_2)$ and heated from the sides. The results are compared with our model study with water, and the qualitative agreement is found to be good.

68 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used a high-speed digital video camera to capture the images of the liquid interface that are illuminated by a laser sheet, with the contrast enhanced by a fluorescent powder in the fluid.
Abstract: A new optical method for the non-intrusive measurement of falling film thickness on the perimeter of horizontal tubes is described. The technique uses a high-speed digital video camera to capture the images of the liquid interface that are illuminated by a laser sheet, with the contrast enhanced by a fluorescent powder in the fluid. The results are compared to those predicted by the Nusselt falling film theory, showing relatively good agreement around the upper perimeter of the tube but much poorer agreement on the lower perimeter. The corresponding effects on heat transfer have also been estimated.

63 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present an overview of the issue, showing basic options to evenly distribute single and two-phase fluid and the methods to determine maldistribution for heat exchangers with microchannels.
Abstract: Heat exchangers with microchannels (hydraulic diameter less than 1 mm) are attracting significant attention lately. Performance of these heat exchangers mostly evaporators is greatly affected by imperfect distribution of a two-phase fluid to each channel. This paper presents an overview of the issue, showing basic options to evenly distribute single and two-phase fluid and the methods to determine maldistribution. Special attention is given to describing two-phase regimes in an adiabatic horizontal header with R134a as a fluid, describing a two-phase map with area of good distribution.

Journal ArticleDOI
TL;DR: In this article, a transient heat conduction model was employed to predict the temperature distribution in the line wall and the solid wax, which was then used to estimate the basic dimensions of the heating coil section and the thermal insulation employed to minimize the heating losses to the cold sea water environment.
Abstract: Total blockage of subsea petroleum production lines due to wax deposition is a relevant problem for the industry. This problem has led to significant capital losses associated with the loss of production and the substitution of plugged lines. The present paper is a study of the feasibility of a remediation procedure aimed at helping the removal of wax plugs. In this procedure, the section of the oil line plugged with wax is inductively heated through an external coil positioned over the line at sea bed. The objective of the work is to estimate the level of electrical power required to soften the wax plug inside the line. To this end, a transient heat conduction model was employed to predict the temperature distribution in the line wall and the solid wax. This information was employed to estimate the basic dimensions of the heating coil section and the thermal insulation employed to minimize the heating losses to the cold sea water environment. A laboratory experimental study with a subsea pipeline section...

Journal ArticleDOI
TL;DR: In this article, local forced and mixed heat transfer coefficients were measured by Ghajar and Tam in a horizontal circular straight tube fitted with reentrant, square-edged, and bellmouth inlets under uniform wall heat flux boundary condition.
Abstract: Local forced and mixed heat transfer coefficients were measured by Ghajar and Tam [5] in a horizontal circular straight tube fitted with reentrant, square-edged, and bell-mouth inlets under uniform wall heat flux boundary condition. For the experiments, the Reynolds, Prandtl, and Grashof numbers varied from about 280 to 49000, 4 to 158, and 1000 to 2.5 × 105, respectively. The heat transfer transition regions were established by observing the change in the heat transfer behavior. The data in the transition region were correlated by using the traditional least squares method. The correlation predicted the transitional data with an average absolute deviation of about 8%. However, about 30% of the data in the transition region were predicted with 10–20% deviation, and about 3% with deviations greater than 20%. This is due to the abrupt change in the heat transfer characteristic and its intermittent behavior in this region. Since the value of the heat transfer coefficient has a direct impact on the size of th...

Journal ArticleDOI
TL;DR: In this paper, the thermal performance of four different arrangements of evaporative air coolers have been evaluated experimentally during the hot summer of Kuwait, and two variables are used to evaluate the system thermal performance, the thermal effectiveness and the energy efficiency ratio (EER).
Abstract: The thermal performance of four different arrangements of evaporative air coolers have been evaluated experimentally during the hot summer of Kuwait. The systems include one-stage direct evaporative cooling (DEC), one-stage indirect evaporative cooler (IEC) linked to an external cooling tower, two-stage indirect/direct evaporative coolers (IEC/DEC), and three-stage system of evaporative cooling and mechanical vapor compression (IEC/DEC-MVC). Two variables are used to evaluate the system thermal performance, the thermal effectiveness and the energy efficiency ratio (EER). The data show that the IEC/DEC has the highest EER, followed by the DEC, IEC/DEC-MVC, and IEC. The DEC has the lowest effectiveness, succeeded by the DEC/IEC, IEC, and IEC/DEC-MVC. Coupling MVC with IEC/DEC extends the cooling range and can cool the ambient air dry bulb temperature over a range of 40°C to 15°C at any value of ambient air wet bulb temperature. Two experimental correlations have been developed for each one of the tested sys...

Journal ArticleDOI
TL;DR: In this article, an advanced capillary structure with high thermal effectiveness, low axial pressure drop, high capillary pressure, and a high boiling limit was developed, which combines open minichannels with open microchannels.
Abstract: High performance heat pipes are widely used for the thermal control of electronic devices. Concerning heat transport limitations, typical wick or capillary structures show advantages in some aspects and disadvantages in others. An advanced capillary structure was developed with high thermal effectiveness, low axial pressure drop, high capillary pressure, and a high boiling limit. It combines open minichannels with open microchannels that are manufactured perpendicular on top of the minichannels. The heat transfer coefficient in the evaporator zone, which is a characteristic value for the thermal effectiveness, was up to 3.3 times higher compared to a similar structure without microchannels. A model that combines micro- and macroscopic phenomena was developed. It predicts the heat transfer coefficient with quite good accuracy as long as the microchannels are at least 300 μm.

Journal ArticleDOI
TL;DR: In this article, the boiling front location was determined from the temperature distribution of the heated wall obtained from liquid crystal thermography, and the impact of various factors on the boiling incipience in microchannels, such as pressure, the inlet liquid subcooling, and flow velocity, were investigated.
Abstract: In this paper, attempts were made to experimentally investigate the boiling incipience in a narrow rectangular vertical channel of 1 mm depth with an external 40 mm wide wall heated uniformly and others assumed quasiadiabatic. The “boiling front” location was determined from the temperature distribution of the heated wall obtained from liquid crystal thermography. Boiling incipience occurs when a considerable rise in the wall temperature above the saturation temperature takes place. Thus, boiling incipience is accompanied by “nucleation hysteresis.” The impact of various factors on the boiling incipience in microchannels, such as pressure, the inlet liquid subcooling, and flow velocity, were investigated.

Journal ArticleDOI
TL;DR: A new graphical thermodynamic rule that avoids the deterioration of energy targets while minimizing the number of heat transfer units as well as the mixing and splitting network complexity has been formalized.
Abstract: This article addresses the problem of minimizing the number of heat exchangers for heat recovery as well as the number of mixing and splitting junctions within water networks while maintaining the energy targets determined by the classical pinch analysis. A new systematic approach is proposed to eliminate the kink points and linearize the composite curves. This is based on a systematic strategy that indicates how to mix and split the water streams in order to modify the shape of the initial composite curves. A new graphical thermodynamic rule that avoids the deterioration of energy targets while minimizing the number of heat transfer units as well as the mixing and splitting network complexity has been formalized. This rule permits the control of the procedure of mixing and splitting on the T-H diagram in order to guarantee the pre-established targets. The proposed approach can be used for either the manual design of heat recovery within water networks or the building of a superstructure with a limited nu...

Journal ArticleDOI
TL;DR: In this paper, the results obtained during an experimental study on film boiling on wires in the presence of an externally imposed electric field were reported, which showed that two different film boiling regimes, separated by an additional boiling transition, can exist.
Abstract: The paper reports the results obtained during an experimental study on film boiling on wires in the presence of an externally imposed electric field. The arrangement allowed achieving various combinations of pressure and subcooling independently of the environmental conditions. The test section was an electrically heated platinum wire with diameters of 0.1 and 0.2 mm. The working fluid was FC-72. The results showed that two different film boiling regimes, separated by an additional boiling transition, can exist in the presence of an electric field. The first regime, at low wire superheat, was strongly influenced by the electric field, showing a remarkable heat transfer enhancement with increasing voltage. The second regime, at higher superheat, was weakly dependent on the field strength and almost coincident with the zero field one. These results are analogous to the ones previously obtained using R113 as working fluid. The reasons for the occurrence of the transition were investigated. A simple model of ...

Journal ArticleDOI
Gian Piero Celata1
TL;DR: In this paper, the authors provide a general overview of single-phase heat transfer and flow in capillary (micro) pipes, and analyze the Laminar-to-Turbulent flow transition in detail.
Abstract: The objective of this paper is to provide a general overview of the research carried out so far in single-phase heat transfer and flow in capillary (micro) pipes. Laminar flow and laminar-to-turbulent flow transition are analyzed in detail in order to clarify the discrepancies among the results obtained by different researchers. Experiments performed in the ENEA laboratory indicate that in laminar flow regime, the friction factor is in good agreement with the Hagen-Poiseuille theory for a Reynolds number below 600–800. For higher values of the Reynolds number, experimental data depart from the Hagen-Poiseuille law to the side of higher f values. The transition from laminar-to-turbulent flow occurs for Reynolds number in the range 1800–2500. Heat transfer experiments show that heat transfer correlations in laminar and turbulent regimes, developed for conventional (macro) tubes, are not properly adequate for heat transfer rate prediction in microtubes.

Journal ArticleDOI
TL;DR: In this paper, a simple, rapid, and robust algorithm for the design of segmentally baffled shell and tube heat exchangers is presented, which ensures full use of the maximum allowable pressure drops given typically as design specifications, with no geometric restrictions on the equipment.
Abstract: A simple, rapid, and robust algorithm for the design of segmentally baffled shell and tube heat exchangers is presented. The algorithm ensures full use of the maximum allowable pressure drops given typically as design specifications, with no geometric restrictions on the equipment. The core of the algorithm is provided by two compact formulations that relate the pressure drop for each side of the exchanger with the film heat transfer coefficient and the exchanger area. The compact formula for the tube side includes the effects of tube ends while the one for the shell side is based on the Bell-Delaware method. The parameters of the compact formulas are used as search variables in the design algorithm. Two examples are presented that show how the algorithm compares to other reported design methods and how the search for realistic solutions is aided by this method.

Journal ArticleDOI
TL;DR: In this article, the authors studied the thermal behavior of plate heat exchangers with both equal and unequal passes of the fluids and showed that the flow maldistribution severely affects the performance of plate-heat exchangers.
Abstract: Over last two decades, plate heat exchangers (PHEs) have presented themselves as a viable alternative to the conventional shell and tube heat exchangers in the process and power industries. The thermal theory available for plate heat exchangers in the literature largely works on the assumption of equal flow in each channel. However, it is well known that the distribution of fluid from port to channel in PHE is far from being uniform. The present study brings about this port to channel flow distribution effect on the thermal behavior of multipass plate heat exchangers. The variation of the heat transfer coefficient due to flow variation from channel to channel has also been taken into consideration. Heat exchangers with both equal and unequal passes of the fluids have been studied. The results indicate that the flow maldistribution severely affects the performance of plate heat exchangers, and multipassing can act as an important tool to reduce the deterioration in performance due to maldistribution. The r...

Journal ArticleDOI
TL;DR: The present design application highlights the GEO features of being easily implemented and efficient on tackling optimization problems when the objective function presents design variables with strong nonlinear interactions and is subject to multiple constraints.
Abstract: In this paper, an application of the Generalized Extremal Optimization (GEO) algorithm to the optimization of a heat pipe (HP) for a space application is presented. The GEO algorithm is a generalization of the Extremal Optimization (EO) algorithm, devised to be applied readily to a broad class of design optimization problems regardless of the design space complexity it would face. It is easy to implement, does not make use of derivatives, and can be applied to either unconstrained or constrained problems with continuous, discrete, or integer variables. The GEO algorithm has been tested in a series of test functions and shows to be competitive to other stochastic algorithms, such as the Genetic Algorithm. In this work, it is applied to the problem of minimizing the mass of an HP as a function of a desirable heat transport capability and a given temperature on the condenser. The optimal solutions were obtained for different heat loads, heat sink temperatures, and three working fluids: ammonia, methanol, and...

Journal ArticleDOI
TL;DR: In this paper, the effects on heat transfer of mounting baffles to the upper inclined surfaces of trapezoidal cavities were investigated. But the authors focused on two thermal boundary conditions: buoyancy assisting mode and buoyancy opposing mode.
Abstract: A numerical investigation examined the effects on heat transfer of mounting baffles to the upper inclined surfaces of trapezoidal cavities. Two thermal boundary conditions are considered. In the first, the left, short vertical wall is heated while the right, long vertical wall is cooled (buoyancy assisting mode along the upper inclined surface of the cavity). In the second, the right, long vertical wall is heated while the left, short vertical wall is cooled (buoyancy opposing mode along the upper inclined surface of the cavity). For each boundary condition, computations are performed for three baffle heights, two baffle locations, four Rayleigh number values, and three Prandtl number values. Results are displayed in terms of streamlines, isotherms, and local and average Nusselt number values. For both boundary conditions, predictions reveal a decrease in heat transfer in the presence of baffles, with its rate generally increasing with increasing baffle height and Prandtl number. For a given baffle height...

Journal ArticleDOI
TL;DR: A numerical simulation using semi-empirical equations of heat and mass transfer performance along the surface of plate condensers was carried out for different multicomponent mixtures with noncondensable components.
Abstract: The application of plate heat exchangers for the condensation of multicomponent mixtures requires reliable, well-grounded methods of calculation. A numerical simulation using semi-empirical equations of heat and mass transfer performance along the surface of plate condensers was carried out for different multicomponent mixtures with noncondensable components. The plates with cross-corrugated patterns for plate condensers were used. The simulation was done for four different types of corrugated plates of industrially manufactured plate heat exchangers. The results of the simulation are in a good accordance with experimental data obtained during long-time experiments for a pilot plant at the pharmaceutical factory in Kharkiv. It is shown that the enhancement of heat and mass transfer in a plate condenser for the case of a four-component mixture gives the possibility of decreasing by 1.8–2 times the necessary heat transfer surface area comparatively with shell-and-tube unit for the same process parameters.

Journal ArticleDOI
TL;DR: In this paper, a special test rig was built in order to observe two-phase flow patterns using a horizontal glass tube with ID 0.98 mm, and flow visualization experiments were conducted for temperatures 20°C and 0°C, for mass flux ranging from 100 to 580 kg m−2 s−1.
Abstract: Carbon dioxide (CO 2 /R-744) is receiving renewed interest as a refrigerant, in many cases for systems with microchannel heat exchangers that have high pressure capability, efficient heat transfer, and compact design. A good understanding of two-phase flow of evaporating CO 2 in microchannels is needed to analyze and predict heat transfer. A special test rig was built in order to observe two-phase flow patterns using a horizontal glass tube with ID 0.98 mm. Flow visualization experiments were conducted for temperatures 20°C and 0°C and for mass flux ranging from 100 to 580 kg m−2 s−1 . The observations showed a dominance of intermittent (slug) flow at low x and wavy annular flow with entrainment of droplets at higher x. The aggravated dryout problem reported from heat transfer experiments at high mass flux could be explained by increased entrainment. The flow pattern observations did not fit generalized maps or transition lines showed in the literature.

Journal ArticleDOI
TL;DR: In this paper, an improved lumped differential approach for ablative thermal protection analysis is proposed, which involves the use of materials with low thermal diffusivity, and the results obtained for a one-dimensional thermal ablation problem in a finite slab are compared against those obtained by previously reported lumped-differential solutions.
Abstract: A computational approach for the engineering analysis of ablative-type thermal protection systems (TPS) in atmospheric reentry ballistic flights is communicated. We first propose an improved lumped differential approach for ablative thermal protection analysis, which involves the use of materials with low thermal diffusivity. The results obtained for a one-dimensional thermal ablation problem in a finite slab are compared against those obtained by previously reported lumped differential solutions. Benchmark results for the local nonlinear model, obtained through the generalized integral transform technique, are utilized to verify the proposed solution in a realistic ablation problem, consisting of a low thermal diffusivity material subjected to a prescribed net aerodynamic heating. In addition, an integrated symbolic–numerical system is constructed based on the Mathematica platform for the derivation and computation of all the related quantities along the flight, yielding the transient behavior of the TPS...

Journal ArticleDOI
Abstract: The numerical solution of the laminar free convection of air around a horizontal cylinder with external longitudinal fins has been reported in this paper. The cylinder surface as well as the surfaces of each fin were assumed to be at a uniform temperature. The fluid drawn over a large angular domain moves out through a narrow, almost vertical strip known as plume, the thickness of which reduces with increasing buoyancy. The heat transfer increases with an increase in Grashof number, the number of fins, and fin length. For a constant fin surface, more fins of lower length result in a better heat transfer for Gr beyond about 10 3 .

Journal ArticleDOI
TL;DR: In this article, the effects of acoustic cavitation and nanometer particles on single-phase convection and boiling heat transfer of a horizontal circular copper tube have been investigated experimentally.
Abstract: Heat transfer characteristics of nanofluids in an acoustic cavitation field have been investigated experimentally The effects of acoustical parameters, nanofluids concentration, and fluid subcooling on heat transfer are determined in detail Results show that acoustic cavitation and nanometer particles have a profound influence on single-phase convection and boiling heat transfer of a horizontal circular copper tube The former is mainly ascribed to the impingement and disturbance of cavitation bubbles and anticipatory activation of smaller vapor embryos within the cavities, while the latter is caused by roughness modification of the tube surface The above mechanisms are given based on the Schlieren photographs and optical observation Acoustic cavitation has been shown to be a good way to reduce or eliminate boiling hysteresis

Journal ArticleDOI
TL;DR: Methods to monitor and control the conditioning film formation and deposit growth for various process industries are found and can function as an alarm device to signal when a critical point of biofilm buildup is reached.
Abstract: This study is concerned with the sequence and character of the events that take place after the initial exposure of the surface to the product, that is, the buildup and composition of a conditioning film on the surface. Calculations show that for short transient studies, conditioning film measurements can be made to values as small as 3.0 microns. Utilizing higher heating rates (5 W/cm2) and shorter transients allow measurements to 2.0 microns. This type of monitoring system can function as an alarm device to signal when a critical point of biofilm buildup is reached. Maintenance procedures can be initiated at the proper time rather than at arbitrary intervals, thus minimizing down time and ensuring product safety. Conclusions include methods to monitor and control the conditioning film formation and deposit growth for various process industries.

Journal ArticleDOI
TL;DR: In this paper, the authors point out that the number of transistors and devices per unit area of the substrate is increasing ferociously, as is the associated power consumed and dissipated in these devices.
Abstract: Are we fooling ourselves? As dutiful engineers and scientists, it’s our job to find solutions to “impossible” problems. But there are some indications that we are approaching a dead end in thermal management. We’ve all heard the story dozens of times before: as the miniaturization of semiconductor electronics continues with no slowdown in sight, the number of transistors and devices per unit area of the substrate is increasing ferociously, as is the associated power consumed and dissipated in these devices. More transistors are being packed into a single chip, more dice into multi-chip modules, and more devices into small confined spaces in systems. In addition, the heat generated by each device is increasing, due to higher operating frequencies, Moore’s Law, etc. This has become such a familiar, repetitive mantra for thermal engineers that we’re in danger of forgetting its true meaning. The truth is that, like it or not, physical systems have practical limits. Someday, maybe soon, we’re going to smack hard into the thermal ceiling. The heat densities in electronic devices are spiraling exponentially upward, but our cooling technologies are not keeping pace. Already, designers are limiting clock speeds and throttling performance for lack of adequate cooling.