Showing papers on "Critical heat flux published in 1999"

Cryogenic heat transfer

Abstract: Presents applied heat transfer principles in the range of extremely low temperatures. The specific features of heat transfer at cryogenic temperatures, such as variable properties, near critical convection, and Kapitza resistance, are described. This book includes many example problems, in each section, that help to illustrate the applications of t

Evaporation Heat Transfer and Pressure Drop of Refrigerant R-134a in a Plate Heat Exchanger

Abstract: Plate heat exchangers (PHE) have been widely used in food processing, chemical reaction processes, and other industrial applications for many years. Particularly, in the last 20 years plate heat exchangers have been introduced to the refrigeration and air conditioning systems as evaporators or condensers for their high efficiency and compactness. Here, the evaporation heat transfer coefficient and pressure drop for refrigerant R-134a flowing in a plate heat exchanger were investigated experimentally in this study. Two vertical counterflow channels were formed in the exchanger by three plates of commercial geometry with a corrugated sine shape of a chevron angle of 60 deg. Upflow boiling of refrigerant R-134a in one channel receives heat from the hot downflow of water in the other channel. The effects of the mean vapor quality, mass flux, heat flux, and pressure of R-134a on the evaporation heat transfer and pressure drop were explored. The quality change of R-134a between the inlet and outlet of the refrigerant channel ranges from 0.09 to 0.18. Even at a very low Reynolds number, the present flow visualization of evaporation in a plate heat exchanger remains turbulent. It is found that the evaporation heat transfer coefficient of R-134a in the plates ismore » much higher than that in circular pipes and shows a very different variation with the vapor quality from that in circular pipes, particularly in the convective evaporation dominated regime at high vapor quality. Relatively intense evaporation on the corrugated surface was seen from the flow visualization. Moreover, the present data showed that both the evaporation heat transfer coefficient and pressure drop increase with the vapor quality. At a higher mass flux the pressure drop is higher for the entire range of the vapor quality but the evaporation heat transfer is clearly better only at the high quality. Raising the imposed wall heat flux was found to slightly improve the heat transfer, while at a higher refrigerant pressure, both the heat transfer and pressure drop are slightly lower. Based on the present data, empirical correlations for the evaporation heat transfer coefficient and friction factor were proposed.« less

Phase change in microchannel heat sinks with integrated temperature sensors

Abstract: A unique technique of mask-less and self-aligned silicon etch between bonded wafers was developed and applied to fabricate a microchannel heat sink integrated with a heater and an array of temperature sensors. The technique allowed the formation of self-aligned and self-stopped etching of grooves between the bonded wafers. The device, consisting of distributed temperature microsensors, allowed direct temperature measurements for different levels of power dissipation under forced convection using either nitrogen or water as working fluids. The measured temperature distributions are used to characterize the micro heat sink performance under forced convection boiling conditions. The onset of critical heat flux (CHF) condition was investigated for different channel sizes and liquid flow-rates. The results suggest that the bubble dynamic mechanism in the microchannel might be different compared with conventional channels.

Experimental Study of the Local Convection Heat Transfer From a Wall-Mounted Cube in Turbulent Channel Flow

Abstract: This paper presents some results of the experimental investigation of the local convective heat transfer on a wall-mounted cube placed in a developing turbulent channel flow for Reynolds numbers between 2750 < ReH < 4970 Experiments were conducted using a specially designed cubic assembly made of heated copper core and a thin epoxy layer on its surface The distribution of the local heat transfer coefficient was obtained from the surface heat flux evaluated from the heat input and computed temperature field in the epoxy layer, and from the surface temperature distribution acquired by infrared thermography In parallel, the flow field was studied using laser doppler anemometer and flow visualizations, aimed at correlating the local heat transfer with the flow pattern and turbulence field The complex vortex structure around the cube, in particular at the top and the side faces, caused large variation in the local convective heat transfer The largest gradients in the distributions of the surface heat transfer were found at locations of flow separation and reattachment Areas of flow recirculation are typically accompanied by a minimum in the heat transfer coefficient It is argued that the local temperature rise of the air in the recirculation zone is caused by the trapped vortex, which acts as an insulation layer preventing the removal of heat from the surface of the cubes In contrast, the intermittent reattachment of the low-temperature shear flow was found to produce large heat transfer coefficients

Heat transfer enhancement of heat exchangers

Abstract: Preface. Introduction to Heat Transfer Enhancement - Preview of Contributions S. Kakac. The Imperative to Enhance Heat Transfer A.E. Bergles. Sustainability Criteria for Heat Exchanger Design N.H. Afgan, M.G. Calvalho. Extended Surface Heat Transfer in Heat Exchangers and Performance Measurements P.J. Heggs. Microfin Tube Technology - The Effects of Spiral Angle on Evaporative Heat Transfer Enhancement S.-Y. Oh, A.E. Bergles. Heat Transfer Enhancement by Wing-Type Longitudinal Vortex Generators and their Application to Finned Oval Tube Heat Exchanger Elements M. Fiebig, Y. Chen. Effect of Fin Heat Conduction on the Performance of Punched Winglets in Finned Oval Tubes Y. Chen, M. Fiebig. Heat Transfer and Fluid Flow in Rib-Roughened Rectangular Ducts B. Sunden. On the Airside Performance of Fin-and-Tube Heat Exchangers C.-C. Wang. Optimum Design of Air-Cooled Fin-and-Tube Heat Exchangers: Accounting for the Effect of Complex Circuiting C.-C. Wang. Flow and Heat Transfer Mechanisms in Plate-and-Frame Heat Exchangers B. Sunden. Heat Transfer Enhancement in a Plate Heat Exchanger with Rib-Roughened Surfaces R. Tauscher, F. Mayinger. Heat Transfer Augmentation in Channels with Porous Copper Inserts T.M. Kuzay, J.T. Collins. Boiling on Structured Surfaces R.L. Webb, L.-H. Chien. Heat Exchangers for Thermoacoustic Refrigerators: Heat Transfer Measurements in Oscillatory Flow C. Herman, M. Wetzel. A Study on the High Performance Ceramic Heat Exchanger for Ultra High Temperatures M. Kumada. Boiling and Evaporation of Falling Film on Horizontal Tubes and its Enhancement on Grooved Tubes Y. Fujita. Numerical and Experimental Investigation of Enhancement of Turbulent Flow Heat Transfer in Tubes by Means of Truncated Hollow Cone Inserts T.Ayhan, et al. Modern Advances in Optical Measuring Techniques -- Tools to Support Energy Conservation F. Mayinger. Enhancement of Combined Heat and Mass Transfer in Rotary Exchangers U. Dinglreiter, F. Mayinger. Advances in Understanding of Flame Acceleration for the Improving of the Combustion Efficiency C. Gerlach, et al. Heat and Mass Transfer with Drying of Water-Based Varnishes J. Mintzlaff, F. Mayinger. Energy Conversion in a Hydrogen Fueled Diesel Engine: Optimization of the Mixture Formation and Combustion P. Prechtl, et al. Enhancement of Heat Transfer with Horizontal Promoters S.U. Onbs oglu, A.N. Egrican. The Effect of Augmented Surfaces on Two-Phase Flow Instabilities S. Kakac. Flow Boiling inside Microfin Tubes: Recent Results and Design Methods J.R. Thome. Flow Boiling of Refrigerant-Oil Mixtures in Plain and Enhanced Tubes J.R. Thome. Influence of Confinement on FC-72 Pool Boiling from a Finned Surface M. Misale, et al. Prediction of Condensation and Evaporation in Micro-Fin and Micro-Channel Tubes R.L. Webb. Performance Enhancement of Heat Exchangers for Semiconductor-Chip Manufacturing Wen-Jei Yang, S. Torii. Evaporation and Condensation Heat Transfer Enhancement for Alternative Refrigerants used in Air-Conditioning Machines T. Ebisu. Development of New Concept Air-Cooled Heat Exchanger for Energy Conservation of Air-Conditioning Machine T. Ebisu. Multi-Hole Cooling Effectiveness on Combustion Chamber Walls B. Leger, P. Andre. Experimental Studies on Influence of Process Variables to the Exergy Losses at the Double Tube Heat Exchangers A. Can, et al. Enhancement of Direct-Contact Heat Transfer in Concentric Annuli T.A. Ozbelge, M.K. Shahidi. U

Boiling crisis and non-equilibrium drying transition

Abstract: Boiling crisis is the rapid formation of the quasi-continuous vapor film between the heater and the liquid when the heat supply exceeds a critical value. We propose a mechanism for the boiling crisis that is based on the spreading of the dry spot under a vapor bubble. The spreading is initiated by the vapor recoil force, a force coming from the liquid evaporation into the bubble. Since the evaporation intensity increases sharply near the triple contact line, the influence of the vapor recoil can be described as a change of the apparent contact angle. Therefore, for the most usual case of complete wetting of the heating surface by the liquid, the boiling crisis can be understood as a drying transition from complete to partial wetting. The state of nucleate boiling, which is boiling in its usual sense, is characterized by a very large rate of heat transfer from the heating surface to the bulk because the superheated liquid is carried away from the heating surface by the departing vapor bubbles. If the heating power is increased, the temperature of the heating surface increases with the heat flux. When the heat flux from the heater reaches a threshold value q CHF (the critical heat flux, CHF), the vapor bubbles suddenly form a film which covers the heating surface and insulates the latter from the bulk of the liquid. The temperature of the heating surface grows so rapidly that the heater can fuse unless its power is controlled. This phenomenon is known under the names of " boiling crisis, " " burnout, " or " Departure from Nucleate Boiling " (DNB) [1]. The final state of this transition is called film boiling. This problem has become very important since the 1940's, with the beginning of the industrial exploitation of heat exchangers with large heat fluxes (as with nuclear power stations). Since then a huge amount of research has been done for the various conditions of pool boiling (boiling without imposed external flow) and flow boiling (boiling of the flowing water) [2]. Numerous empirical correlations have been proposed, each describing the dependence of the CHF on the physical parameters of the liquid and of the heater more or less correctly for a particular geometry and particular conditions of boiling [2]. A strong dependence of the threshold on the details of the experimental setup coupled with difficulties in separating the consequences of DNB from its causes is at the origin of a large number of frequently controversial hypotheses [2]. The violence of boiling makes observations quite difficult. Good quality photographic experiments are presented in only a few articles (see e.g. [3] – [6]). Despite an increasing interest in the physical aspect of the problem during recent years [7,8] and numerous empirical approaches, the underlying physics still remains obscure. In this Letter, we propose a model based on a non-equilibrium drying transition.

Heat transfer from catalysts with computational fluid dynamics

Abstract: A computational fluid dynamics software package was used to study heat transfer from spherical particles of different sizes and under different heat-transfer conditions. It was shown that although the Ranz-Marshall and other similar correlations are valid in the case where particles do not interact, this is not true for densely packed systems such as those that we find in reactors commonly used in olefin polymerization. It was also demonstrated that convection is in fact not the only means of removing heat from small, highly active particles. Conductive heat transfer between large and small particles present in the same reactor appears to help alleviate problems of overheating and explain why earlier models of heat transfer in olefin polymerization overpredict the temperature rise during early stages of polymerization.

Low-flow Critical heat flux in heated microchannels

Abstract: Critical heat flux (CHF) associated with the flow of subcooled water in heated microchannels is experimentally investigated. Four different channels, all 16 cm in length, are used: two are circular...

High Resolution Measurements of Local Heat Transfer Coefficients by Discrete Hole Film Cooling

Abstract: Local heat transfer coefficients on a flat plate surface downstream a row of cylindrical ejection holes were investigated The parameters blowing angle, hole pitch, blowing rate, and density ratio were varied over a wide range, emphasizing engine relevant conditions A high-resolution IR-thermography technique was used for measuring surface temperature fields Local heat transfer coefficients were obtained from a Finite Element analysis IR-determined surface temperatures and backside temperatures of the cooled test plate measured with thermocouples were applied as boundary conditions in this heat flux computation The superposition approach was employed to obtain the heat transfer coefficient h f based on the difference between actual wall temperatures and adiabatic wall temperatures in the presence of film cooling The h f data are given for an engine relevant density ratio of 18 Therefore, heat transfer results with different wall temperature conditions and adiabatic film cooling effectiveness results for identical flow situations (ie, constant density ratios) were combined Characteristic surface patterns of the locally resolved heat transfer coefficients h f are recognized and quantified as the different ejection parameters are changed The detailed results are used to discuss the specific local heat transfer behavior in the presence of film cooling They also provide a base of surface data essential for the validation of the heat transfer capabilities of CFD codes in discrete hole film cooling

Enhanced Heat Transfer: Endless Frontier, or Mature and Routine?

Abstract: This paper considers the many techniques that have been developed to enhance convective heat transfer. They are presented according to the mode of heat transfer. The current advanced enhancement represents 3rd generation heat transfer technology. Many studies of Prof. Franz Mayinger are integral to this development. It is felt that this field has a bright future.

Temperature Response of Heat Transport in a Micro Heat Pipe

Abstract: A detailed mathematical model for predicting the heat transport capability and temperature gradients that contribute to the overall axial temperature drop as a function of heat transfer in a micro heat pipe has been developed. The model utilizes a third-order ordinary differential equation, which governs the fluid flow and heat transfer in the evaporating thin film region; an analytical solution for the two-dimension heat conduction equation, which governs the macro evaporating film region in the triangular corners; the effects of the vapor flow on the liquid flow in the micro heat pipe; the flow and condensation of the thin film caused by the surface tension in the condenser; and the capillary flow along the axial direction of the micro heat pipe. With this model, the temperature distribution along the axial direction of the heat pipe and the effect on the heat transfer can be predicted. In order to verify the model presented here, an experimental investigation was also conducted and a comparison with experimental data made

Essential Heat Transfer

Abstract: Introduction. Conduction - Analytical Methods and Applications. Conduction - Numerical Techniques and Application. Principles of Convection. Forced Convection. Free Convection. Condensation and Boiling Heat Transfer. Radiative Heat Transfer. Heat Exchangers. Heat Transfer Instrumentation.

Evaporative Heat Transfer in a Capillary Structure Heated by a Grooved Block

Abstract: Experimental results of capillary-driven heat and mass transfer in a vertical rectangular capillary porous structure heated from a grooved block placed on the top are reported in this paper. The formation of the liquid-vapor menisci in the vicinity of the downward-facing heated surface provided the capillary-force for the upflow of water in the porous structure. The temperature distributions in both the heating block and the porous structure as well as the induced mass flow rate of water were measured under different heat flux conditions. The experimental results show that with an increase of the imposed heat flux, the heat transfer coefficient increases to a maximum value and then decreases afterwards. It is also found that the liquid-vapor interface moved towards the downward-facing heated surface as the imposed heat flux was increased. The heat transfer mechanisms leading to the maximum heat transfer coefficient and the critical heat flux are explained based on the visual observation of the phasechange behavior and the measured temperature distributions within the porous structure. The effects of particle sizes, the inlet temperature of the subcooled liquid, and the adverse gravity force on the heat transfer characteristics are also examined.

Heat transfer column for geothermal heat pumps

Abstract: An improved heat transfer column for geothermal heating and cooling in which refrigerant lines are wound about a thin wall vertically oriented tube. The tube is surrounded by a flexible liner which is filled with water, the liner being positioned within a cavity in an earth mass. During operation, water within the tube rises by convection and transfers heat evenly to the water within the liner for subsequent transfer to the earth mass. This heat is subsequently extracted from the earth mass during a heating cycle, and means is provided for transfer of a portion of the heat in the water to the ambient atmosphere.

Pool Boiling Critical Heat Flux in Reduced Gravity

Abstract: An experimental investigation has been conducted to measure pool boiling critical heat fluxes in reduced gravity. A horizontal cylindrical cartridge heater immersed in water at reduced pressures during parabolic flights on NASA's KC-135 resulted in boiling on the heater surface. Visual observations and qualitative data trends indicate that the conventional Taylor-Helmholtz instability model still governs the critical heat flux mechanism over the range of gravitational accelerations of the current study, which range from 0.0005 < g/g o < 0.044. Using data from more than 40 individual tests, two semi-empirical correlations have been developed to account for the effect of thermocapillary flow, which tends to decrease the critical heat flux below the predictions of previous correlations