Showing papers on "Heat transfer coefficient published in 1997"

Heat Transfer: A Practical Approach

Abstract: Part 1 Fundamentals: basic concepts of thermodynamics and heat transfer heat conduction steady heat conduction transient heat conduction numerical methods in heat transfer forced convection natural convection boiling and condensation radiation heat transfer heat exchangers mass transfer. Part 2 Applications: heating and cooling of buildings refrigeration and freezing of foods cooling of electronic equipment property tables and charts (SI units and English units) about the software.

Heat Exchangers: Selection, Rating, and Thermal Design

Abstract: CLASSIFICATIONS OF HEAT EXCHANGERS Introduction Recuperation and Regeneration Transfer Processes Geometry of Construction Heat Transfer Mechanisms Flow Arrangements Applications Selection of Heat Exchangers BASIC DESIGN METHODS OF HEAT EXCHANGERS Introduction Arrangement of Flow Path in Heat Exchangers Basic Equations in Design Overall Heat Transfer Coefficient The LMTD Method for Heat Exchangers Analysis The e-NTU Method for Heat Exchangers Analysis Heat Exchanger Design Calculation Variable Overall Heat Transfer Coefficient Heat Exchanger Design Methodology FORCED CONVECTION CORRELATIONS FOR SINGLE-PHASE SIDE OF HEAT EXCHANGERS Introduction Laminar Forced Convection The Effect of Variable Physical Properties Turbulent Forced Convection Turbulent Flow in Smooth Straight Noncircular Ducts The Effect of Variable Physical Properties in Turbulent Forced Convection Summary of Forced Convection in Straight Ducts Heat Transfer from Smooth-Tube Bundles Heat Transfer in Helical Coils and Spirals Heat Transfer in Bends HEAT EXCHANGER PRESSURE DROP AND PUMPING POWER Introduction Tube-Side Pressure Drop Pressure Drop in Tube Bundles in Cross-Flow Pressure Drop in Helical and Spiral Coils Pressure Drop in Bends and Fittings Pressure Drop for Abrupt Contraction, Expansion, and Momentum Change Heat Transfer and Pumping Power Relationship FOULING OF HEAT EXCHANGERS Introduction Basic Considerations Effects of Fouling Aspects of Fouling Design of Heat Exchangers Subject to Fouling Operation of Heat Exchangers Subject to Fouling Techniques to Control Fouling DOUBLE-PIPE HEAT EXCHANGERS Introduction Thermal and Hydraulic Design of Inner Tube Thermal and Hydraulic Analysis of Annulus Parallel-Series Arrangements of Hairpins Total Pressure Drop Design and Operational Features DESIGN CORRELATIONS FOR CONDENSERS AND EVAPORATORS Introduction Condensation Film Condensation on a Single Horizontal Tube Film Condensation on Tube Bundles Condensation Inside Tubes Flow Boiling SHELL-AND-TUBE HEAT EXCHANGERS Introduction Basic Components Basic Design Procedure of a Heat Exchanger Shell-Side Heat Transfer and Pressure Drop COMPACT HEAT EXCHANGERS Introduction Heat Transfer and Pressure Drop THE GASKETED-PLATE HEAT EXCHANGERS Introduction Mechanical Features Operational Characteristics Passes and Flow Arrangements Applications Heat Transfer and Pressure Drop Calculations Thermal Performance CONDENSERS AND EVAPORATORS Introduction Shell and Tube Condensers Steam Turbine Exhaust Condensers Plate Condensers Air Cooled Condensers Direct Contact Condensers Thermal Design of Shell-and-Tube Condensers Design and Operational Considerations Condensers for Refrigeration and Air Conditioning Evaporators for Refrigeration and Air Conditioning Thermal Analysis Standards for Evaporators and Condensers APPENDICES Physical Properties of Metals and Nonmetals Physical Properties of Air, Water, Liquid Metals, and Refrigerants Each chapter also contains sections of Nomenclature, References, and Problems

Convective and radiative heat transfer coefficients for individual human body segments

Abstract: Human thermal physiological and comfort models will soon be able to simulate both transient and spatial inhomogeneities in the thermal environment With this increasing detail comes the need for anatomically specific convective and radiative heat transfer coefficients for the human body The present study used an articulated thermal manikin with 16 body segments (head, chest, back, upper arms, forearms, hands, pelvis, upper legs, lower legs, feet) to generate radiative heat transfer coefficients as well as natural- and forced-mode convective coefficients The tests were conducted across a range of wind speeds from still air to 50 m/s, representing atmospheric conditions typical of both indoors and outdoors Both standing and seated postures were investigated, as were eight different wind azimuth angles The radiative heat transfer coefficient measured for the whole-body was 45 W/m2 per K for both the seated and standing cases, closely matching the generally accepted whole-body value of 47 W/m2 per K Similarly, the whole-body natural convection coefficient for the manikin fell within the mid-range of previously published values at 34 and 33 W/m2 per K when standing and seated respectively In the forced convective regime, heat transfer coefficients were higher for hands, feet and peripheral limbs compared to the central torso region Wind direction had little effect on convective heat transfers from individual body segments A general-purpose forced convection equation suitable for application to both seated and standing postures indoors was hc=103v06 for the whole-body Similar equations were generated for individual body segments in both seated and standing postures

A Detailed Analysis of Film Cooling Physics: Part III — Streamwise Injection With Shaped Holes

Abstract: The physics of the film cooling process for shaped, streamwise-injected, inclined jets is studied for blowing ratio (M = 1.25, 1.88), density ratio (DR = 1.6), and length-to-diameter ratio (L/D = 4) parameters typical of gas turbine operations. A previously documented computational methodology is applied for the study of five distinct film cooling configurations: (1) cylindrical film hole (reference case); (2) forward-diffused film hole; (3) laterally diffused film hole; (4) inlet shaped film hole, and (5) cusp-shaped film hole. The effect of various film hole geometries on both flow and thermal field characteristics is isolated, and the dominant mechanisms responsible for differences in these characteristics are documented. Special consideration is given to explaining crucial flow mechanisms from a vorticity point of view. It is found that vorticity analysis of the flow exiting the film hole can aid substantially in explaining the flow behavior downstream of the film hole. Results indicate that changes in the film hole shape can significantly alter the distribution of the exit-plane variables, therefore strongly affecting the downstream behavior of the film. Computational solutions of the steady, Reynolds-averaged Navier-Stokes equations are obtained using an unstructured/adaptive, fully implicit, pressure-correction solver. Turbulence closure is obtained via the high-Reynolds-number {kappa}-{epsilon}more » model with generalized wall functions. Detailed field results as well as surface phenomena involving adiabatic film effectiveness {eta} and heat transfer coefficient (h) are presented. When possible, computational results are validated against corresponding experimental cases from data found in the open literature. Detailed comparisons are made between surface and field results of the film hole shapes investigated in this work; design criteria for optimizing downstream heat transfer characteristics are then suggested.« less

Evaluation of Manometric Temperature Measurement as a Method of Monitoring Product Temperature During Lyophilization

Abstract: The objective of this study was to evaluate manometric temperature measurement as a non-invasive method of monitoring product temperature during the primary drying phase of lyophilization. This method is based on analysis of the transient response of the chamber pressure when the flow of water vapor from the chamber to the condenser is momentarily interrupted. Manometric temperature measurements (MTM) were compared to product temperature data measured by thermocouples during the lyophilization of water, mannitol, lactose and potassium chloride solutions. The transient pressure response was mathematically modeled by assuming that four mechanisms contribute to the pressure rise: 1) direct sublimation of ice through the dried product layer at a constant temperature, 2) an increase in the temperature at the sublimation interface due to equilibration of the temperature gradient across the frozen layer, 3) an increase in the ice temperature due to continued heating of the frozen matrix during the measurement, and 4) leaks in the chamber. Experimental transient pressure response data were fitted to an equation consisting of the sum of these terms containing three variables corresponding to the vapor pressure of ice, product resistance to vapor flow, and the vial heat transfer coefficient. Excellent fit between the mathematical model and the experimental data was observed, and the value of the variables was calculated from the measured transient pressure response by a least squares method. The product temperature measured by MTM, which measures the temperature at the sublimation interface, was compared with product temperature measured by thermocouples placed in the bottom center of the vials. Manometrically measured temperatures were consistently lower than the thermocouple measurements by about 2 degrees C, this difference being largely accounted for by the temperature gradient across the frozen layer. The resistance of the dried product to mass transfer calculated from MTM was found to agree reasonably well with values measured by a direct vial technique. Product resistance was observed to increase with increasing solute concentration, and to increase continuously as the depth of the dried product layer increases for mannitol and potassium chloride. For lactose, product resistance increases continuously with thickness up to the onset of collapse, at which point the product resistance becomes essentially independent of depth. Scanning electron microscopy was used to explain this observation based on changes in morphology of the solid. The vial heat transfer coefficients obtained from regression analysis were on the order of 10(-3)-10(-4) cal.sec-1. degrees C-1; however, the scatter in the vial heat transfer coefficient data prevents the method from being used for accurate measurement of the vial heat transfer coefficient. The results of the study show that the manometric method shows promise as a process development tool and as an alternative method of in-process product temperature measurement during primary drying.

Numerical analysis of natural convection with surface radiation in a square enclosure

Abstract: The interaction of natural convection with thermal radiation of gray surfaces in a square enclosure filled with air has been numerically investigated. The effect of radiation on the flow field, temperature distribution, and heat transfer is predicted. The result shows that surface radiation significantly altered the temperature distribution and the flow patterns, especially at higher Rayleigh numbers. The average convection Nusselt number increases with the increase of Ra. The presence of surface radiation can change the value of average convection Nusselt number, but only little variation can be observed with the increase of emissivity. The average radiative Nusselt number rises quickly with the increase of emissivity, and radiation heat transfer plays an important part in overall heat flux at larger emissivity. The correlation of entire average Nusselt number has also been discussed for evaluating heat transfer through the enclosure,

Numerical Simulation of Saturated Film Boiling on a Horizontal Surface

Abstract: The past efforts in applying linear Taylor instability theory to the prediction of heat transfer during film boiling on a horizontal surface have suffered from the fact that empirical correlations must be used to define the shape of vapor-liquid interfaces and to determine the transport of mass and heat across these interfaces. The objective of this study is to clarify the physics of film boiling and to predict heat transfer coefficients through complete numerical simulation of the evolving interface between superposed layers of immiscible fluids. A coordinate transformation technique supplemented by a numerical grid generation method and a second-order projection method are combined to solve for the flow and temperature fields associated with an evolving interface. From the numerical simulation, the film thickness and, in turn, the heat transfer coefficient are found to vary both spatially and temporally. Increased wall superheat not only thickens the vapor film in the valley but also enlarges the vapor bulge. The effect of increased system pressure is to slow down the growth of the interface.

Two-phase flow in microchannels

Abstract: : The purpose of this study was to investigate fluid mechanic and heat transfer characteristics of two-phase two-component flow in rectangular microchannels Experiments were conducted using rectangular aluminum channels with hydraulic diameters ranging between 56 micrometers and 256 micrometers and aspect ratios which varied from 05 to 15 Both single- and two-phase tests were conducted using water and gaseous argon, helium, and nitrogen as the working fluids The Reynolds number for both types of experiments ranged from approximately 50 to nearly 10,000 The Nusselt number ranged between 00002 and 70 The single- and two-phase experimental data were empirically correlated, using parameters derived from a dimensional analysis Experimental data were also used to correlate the unknown variables in derived analytical expressions Both single- and two-phase tests yielded excellent correlations of the friction factor For Nusselt number, the correlations were fair to poor Reynolds number and the combination of Reynolds number and Prandtl number were the dominant parameters in the prediction of pressure drop and heat transfer rate, respectively, in both single- and two-phase flows The pressure drop predictions based on the semi-empirical relations by Martinelli for two-phase flows were shown to substantially over-predict the pressure drop measured in these experiments

Temperature Variation and Heat Transfer in Triangular Grooves with an Evaporating Film

Abstract: A mathematical model for the evaporating heat transfer coefe cient and temperature variation along the axial direction of a grooved plate has been developed. The model includes the effects of the capillaryinduced e ow in the grooves caused by the surface tension, the two-dimensional heat conduction occurring in the wall and liquid e lm, and the e ow and evaporation of the thin e lm caused by the disjoining pressure and surface tension in the thin e lm region. The results obtained from this model indicate that if a constant heat e ux boundary condition is applied, the heat transfer coefe cient will decrease and the wall temperature will increase along the axial direction. In addition, the apparent contact angle increases with increasing superheat, and hence, the heat transfer rate through the micro region also increases along the axial direction. Finally, it has been demonstrated that the contribution of the surface tension variation caused by temperature increases in the micro region can be neglected. This work will lead to a better understanding of the axial heat transfer coefe cient and temperature distribution on grooved surfaces, and how these parameters affect the e lm thickness.

Heat Transfer and Hydrodynamics in a Three-Phase Slurry Bubble Column

Abstract: The instantaneous and time-averaged heat transfer coefficients in the regions near the wall and at the center and average gas holdups were measured in a 0.28 m diameter slurry bubble column for the air−water and air−water−glass beads (35 μm) system. The effects of high gas velocities (up to 0.35 m/s) and high solids concentrations (up to 40 vol %) were investigated. Gas holdup decreased with increasing slurry concentrations; the rate of decline was rapid at high gas velocities. The instantaneous local heat transfer measurements were analyzed to study the bubble behavior in the regions near the wall and at the center for different solids concentrations. Larger bubbles were detected in the wall region in slurry systems compared to the solid-free system. The average heat transfer coefficient decreased with increasing slurry concentrations. The heat transfer coefficient was always lower at the wall than at the center.

Numerical Study of Heat Transfer and Flow of Natural Convection in AN Enclosure with a Heat-Generating Conducting Body

Abstract: A numerical study has been conducted to investigate steady state heat transfer and flow characteristics of natural convection in a vertical square enclosure when a temperature difference exists across an enclosure and, at the same time, a conducting body generates heat within the enclosure. Dimensionless governing equations indicate that the heat transfer and flow characteristics of this system are governed by the Rayleigh and Prandtl numbers, the area ratio, the conductivity ratio, and the temperature-difference ratio. Here the temperature-difference ratio is defined as the ratio of a temperature difference across the enclosure to that caused by the heat source. In the present study, the Rayleigh number ranges from 103 to 104, and the temperature-difference ratio from 0 to 50, while the Prandtl number, the area ratio, and the conductivity ratio are kept constant at 0.71, 0.25, 1, respectively. The analysis is performed by observing variations of streamlines, isotherms, heatlines, and the average Nusselt ...

Theoretical and Experimental Study of Electrohydrodynamic Heat Transfer Enhancement Through Wire-Plate Corona Discharge

Abstract: Heat and mass transfer between a surface and the surrounding gas can be enhanced by the application of electric body forces that induce jet or plume-like fluid motion. Such enhancement causes no noise or vibration, can be applied in complex, isolated geometries, and allows simple control of surface temperatures. This paper examines the potentially useful case of multiple fine-wire electrodes suspended in the open air above a grounded and heated horizontal surface. An infrared camera system was used to obtain a complete and accurate distribution of local heat transfer coefficients. on the impingement surface. A numerical code was developed and verified by comparison with experimental data. This code was then used to investigate and compare the heat transfer generated by novel electrode geometries.

Experimental study on forced convective boiling heat transfer of pure refrigerants and refrigerant mixtures in a horizontal tube

Abstract: Convective boiling heat transfer coefficients of pure refrigerants (R22, R32, R134A, R290, and R600a) and refrigerant mixtures (R32R134a, R290R600a, and R32R125) are measured experimentally and compared with Gungor and Winterton correlation. The test section is made of a seamless stainless steel tube with an inner diameter of 7.7 mm and is uniformly heated by applying electric current directly to the tube. The exit temperature of the test section was kept at 12°C ± 0.5°C for all refrigerants in this study. Heat fluxes are varied from 10 to 30 kW m−2 and mass fluxes are set to the discrete values in the range of 424–742 kg m−2 s−1 for R22, R32, R134a, R32R134a, and R32R125; 265–583 kg m−2 s−1 for R290, R600a, and R290R600a. Heat transfer coefficients depend strongly on heat flux at a low quality region and become independent as quality increases. The Gungor and Winterton correlation for pure substances and the Thome-Shakil modification of this correlation for refrigerant mixtures overpredicts the heat transfer coefficients measured in this study.

Evaluation of heat transfer and thermal stability of supercritical jp-7 fuel

Abstract: A series of electrically heated tube experiments was conducted to investigate the potential of JP-7 as a coolant under conditions relevant to a Mach 8 propulsion system. The heat transfer capabilities, carbon deposition, and material compatibility of JP-7 at surface temperatures up to 1700 F (927 C) were tested in 0.125 in. diameter tubes of 304 SS, Inconel 617, Haynes 188, Haynes 230, and 50150 Moly-Rhenium. The heat transfer to the coolant was modeled well by a Dittus-Boelter correlation at lower heat fluxes. At higher heat fluxes, audible instabilities were observed and corresponded to a significant enhancement in the coolant heat transfer. The carbon deposition rates in these tests were comparable to those in previous experiments at lower heat fluxes and much longer residence times. This result suggests that alternative paths of the deposition mechanism may be enhanced under high heat flux test conditions. Microscopic investigation of the post test tubes indicated that there was a significant layer of ordered carbon deposits that had not been seen in the tests at lower heat flux.