Showing papers in "Heat Transfer Engineering in 1992"

Flow Boiling Heat Transfer in Vertical Tubes Correlated by an Asymptotic Model

Abstract: In flow boiling, the nucleate and convective components are superimposed by a very complex mechanism, which so far is not well understood Two models exist in present literature, one by Chen [3] (1963), using addition of the two components with a suppression factor; and one by Shah [8] (1976), using the “greater of” the two components with a Bo-number simplified correlation Neither model presents a satisfactory solution, as attested by the numerous methods published since then, mostly based only on regression analysis-derived correction factors In this article a new model, based on asymptotic addition of the two boiling components, is introduced It follows the established principles of flow boiling and converges correctly to the extremes of all parameters Tested on the University of Karlsruhe data bank containing over 13,000 data points in vertical flow boiling, results superior to previous correlations are demonstrated

A Critical Review of Correlations for Convective Vaporization in Tubes and Tube Banks

Abstract: Three types of phenomenological models are used for calculating heat transfer coefficients in convective vaporization—the superposition, asymptotic, and enhancement models. These models were originally proposed for in-tube vaporization. The objective of the present study is to critically assess these models, and to establish a rational basis for a modified model. Saturated convective vaporization is of key interest in this review. Much more work has been done on in-tube flow than for tube banks. This article surveys the currently available models, and establishes the commonality and differences of the correlations. Separate emphasis is given for tube bank correlations. The correlations for tube banks are compared to those used for in-tube flow. We show that the current correlations may be grouped into three basic types—superposition, enhancement, and asymptotic models. Each type is discussed in order to address the concepts involved in them. Problems associated with certain published models are d...

Two-Dimensional Effects and Design Criteria for Convective Extended Surfaces

Abstract: An analysis of the thermal performance of convective extended surfaces is presented that takes into account the primary function of the fins, which is to augment the heat transfer. The measure of the heat transfer augmentation is expressed by the removal number Nr, which we proposed should be O(10). It is shown that in order to satisfy this criterion the transverse Biot number hw/k must be of the order of 0.01. Further, it is argued that the fin is fully utilized if the aspect number is O(1). These criteria, Bi; = O(0.01) and u = 0(1), offer guidelines for designing fins and are applicable to longitudinal, annular, and pin fins, of uniform thickness or tapered profile. They must substitute the criterion that is frequently used, Bi < I, which was derived from the analysis of longitudinal fins. A very important consequence of this investigation is that fins designed to meet the above criteria can be analyzed using the classical one-dimensional method without introducing any appreciable error.

Thermodynamics of Adsorption Cycles: A Theoretical Study

Abstract: Description of performance of inverse cycle machines using adsorption-desorption processes generally involves different modeling techniques with various simplifications owing to the temperature dependence of the physical properties The corresponding results can be significantly different The objective of this article is to present a complete and precise thermal model (reference model) using variable physical properties with temperature and particularly latent heat of boiling, isosteric heat of desorption, and specific heat of the working pairs We compare, for two major criteria (coefficient of performance and cycled mass) and for two working pairs (methanol, ethanol, active charcoals), the accuracies of the various models cited in the literature and the consequences of the simplifying assumptions

Analysis of spray-cooled finned-tube heat exchangers

Abstract: A mathematical model has been developed for the analysis of spray-cooled finned-tube heat exchangers. An experimental study was conducted on a four-pass, finned-tube heat exchanger in a vertical air/water mist flow to validate the model, and the results compared well with the predicted performance. Significant performance enhancement (up to 3.5 times the dry performance) was found by spraying relatively small amounts of water onto the heat exchanger. The two-phase pressure drop across the heat exchanger was also measured, and the spray water mass flow rate was found to have a significant effect on the pressure drop across the tube bundle. This study helped to identify certain factors, such as the geometry of the finned tubes and the optimum air/spray water ratios, which have to be taken into consideration when designing spray-cooled heat exchangers.

Frictional pressure drop for gas/liquid two-phase flow in plate heat exchangers

Abstract: Two-phase flow occurs in plate heat exchangers used as evaporators or condensers. The frictional pressure drop for two-phase flow can be considerably higher than for single-phase flow, because of the interaction between the phases. In this article, measurements of air-water flow in two different plate-and-frame exchangers are presented. Correlations developed for tubular flow are compared with the experimental results. Recommendations for a calculation procedure are given.

Two-Dimensional Transient Heat Conduction Analysis Using Spreadsheets

Abstract: A method for using spreadsheets to perform a numerical analysis of one- and two-dimensional transient heat conduction is described. The numerical method is based on the explicit nodal equations and is described in terms of the thermal resistance and capacitance formulation. The spreadsheet setup uses an iteration macro and can use either a single-layer spreadsheet or multiple-layer linked spreadsheets. The method allows the use of various boundary conditions, including a specified surface temperature, insulated surfaces, a convective heal transfer coefficient distribution on any surface, and an equivalent radiation heat transfer coefficient. A variable grid can be used that allows a finer grid in areas of high gradients. Thermal properties may be heterogeneous and temperature dependent. The results are highly visual and can be easily plotted in two- and three-dimensional graphs.

Basic Principles of Microwave Power Heating

Abstract: This article outlines the various parameters that are employed in the design of a microwave power applicator as well as the physical principles of the microwave energy conversion process that underlie the optimization of energy consumption and product quality.

Mechanisms by which fouling can increase overall heat transfer coefficients

Abstract: Two probable mechanisms exist whereby the overall coefficient of a heat exchanger can be increased when fouling occurs. These are the creation of roughness elements and restrictions to flow, both of which increase film heat transfer coefficients. The change ( increase/decrease) in the overall heat transfer coefficient due to the formation of deposits on a heat transfer surface is dependent on the relative magnitude of many design and operating variables. In this article, the effect of surface roughness changes, based on an earlier scientific study, is briefly described. Whether the formation of rough deposits can effect an increase in thermal performance, which might create the impression that a negative fouling resistance has arisen, depends critically on the flow Reynolds number and the nature of the roughness. The effect of flow restrictions is demonstrated by a simple mathematical analysis. Whether a flow restriction due to fouling can cause an increase in thermal performance is governed by the magnit...

Experimental investigation of single-phase, condensation, and flow boiling heat transfer for a spirally fluted tube

Abstract: A spirally fluted tube was tested in a vertically oriented shell-and-tube heat exchanger in both condensing and evaporating service. The heat exchanger showed substantial improvement in performance over that expected from a comparable exchanger with plain tubes. The condensation tests were run with working fluids of ammonia and R-114 condensing on the shell side of the heat exchanger. The boiling tests were run with R-11 boiling on the tube side in upward forced convection; heat was supplied by steam condensing on the shell side.

Reducing Particle Deposition in Air-Cooled Heat Exchangers

Abstract: An investigation has been made into reducing particle deposition on the gas side of extended-surface heat exchanger surfaces by using six different “spoiler” designs, i.e., wedge, small rod, large rod, plate, profin, and coiled coil “spoilers.” The effects on fouling resistance, Colburn j factor, friction factor, and fan power have been determined experimentally and developed into characteristic equations. The investigation showed that the tendency of particle deposition can be reduced by using an appropriate “spoiler” design. The effect of a “spoiler” is influenced by its position and air velocity, but apparently not by particulate bulk concentration.

The Effect of a Gap Between Layers on the Heat Transfer Performance of Aligned Tube Banks

Abstract: An s1/d = 3, s2/d = 1.1 aligned tube bank with different gap widths between lube layers was employed for investigation. The value s3/d varied between 1.2 and 6 and the Reynolds number varied from 3,000 to 10,000, which covers the range of practical interest. Tube layers consisting of three or four tubes have been tested and the experimental data were compared with the standard aligned lube bank. Experimental data showed that the existence of a gap enhanced the heat transfer of those tubes just adjacent to the gap by 10-30%, and the heat transfer was fully developed again at the fourth row behind the gap. Therefore these data could be used to calculate the heat transfer coefficient of aligned tube banks with gaps between tube layers having any number of rows.

Historical Development of the Theory of Heat and Thermodynamics: Review and Some Observations

Abstract: This article traces the historical development of the basic concepts in thermodynamics from the time of Galileo (1592) to the time of Planck (1900) to gain historical perspectives. The review of the theory of heat as a branch in physics and thermodynamics is based on secondary sources. Some observations are made regarding the analogous physical phenomena in conduction and convection heat transfer problems. It is pointed out that Newton's cooling problem (1701) is closely related to Graetz's thermal entrance region problem (1885), and Prandtl's boundary-layer concept (1904) can be traced to transient heat conduction problems involving semiinfinite solids. It is also noted that the heat pipe (1964) represents a limiting case of the Carnot cycle (1824) with zero power output.

Negatively Buoyant Flow Along Vertical Cylinders at High Rayleigh Numbers

Abstract: Conditions that must be satisfied before the launch of a Space Transportation System (STS) include combinations of ambient temperature and wind speed that are intended to prevent ice formation on the External Tank (ET) and a local air temperature around the STS of 33°F (0.56°C) or less. Nineteen effluent gases are purged or vented from the STS into the launch pad environment, the most significant of these being the cold and negatively buoyant boil-off gaseous oxygen (GOX) from the oxygen tank. Vented into the ambient, this GOX can be wind-carried toward the STS, where it will cool the launch pad environment. This article described a combined three-dimensional (3D) and two-dimensional (2D) computational fluid dynamics (CFD) analysis that investigated this cooling effect. The results of this analysis for a 6-knot (3.09 m/s) west wind and an ambient temperature of 38°F (3.33°C) indicate that the GOX could cool, relative to the ambient, the local air in the vicinity of the STS and the surface of the east Rede...

Condensate Subcooling near Tube Exit During Horizontal In-Tube Condensation

Abstract: An investigation has been carried out on condensate subcooling near the tube exit during horizontal in-tube condensation, both experimentally and analytically. In the experiment, test fluids used were Refrigerants R113, R11, R114, and R12. Condensate subcoolings of larger than 10 K were measured, even before condensation was completed, and data of condensate temperature profiles were obtained along the adiabatic section following the condensing section. Analysis, based on a nonequilibrium heat transfer model, could closely predict experimental data both for condensate subcooling at the condensing section exit and for condensate temperature profiles in the adiabatic section.

Improving Heat Transfer with Pool Boiling by Covering of Heating Surface with Metallic Spheres

Abstract: In this paper, boiling heat transfer (BHT) is investigated experimentally. Smooth copper walls were covered with single sphere layer and corresponding temperature difference and heat flux were measured. The results were compared with published data for several types of heating surfaces. Comparative analysis shows that surfaces covered with spheres have characteristics as good as the other systems, if not better. There are many ways to enhance boiling heat transfer. One of them is to cover the heating surface with a layer of solid particles, which either remain on the surface during the process or circulate through the boiling liquid, generating a porous two-component, three-phase system. Particles are made of various materials (glass, alumosilicate, corundum, sand, mullite some metals, etc.), which are shaped as spheres mostly, but sometimes are irregular bodies. Many different parameters were proposed to characterize the porous layer. The influence of particles can be expressed by introducing the effective thermal-physical properties of a complex medium. Also, if the working regime can be described as any kind of fluidization, then all quantities developed to be applied to this matter can be used in the case of heat fluidization.

Two-Dimensional Thermal Analysis of a Robotic Arm and Hand with Insulation Interacting with a High-Temperature Field

Abstract: Two-dimensional thermal analysis of the effect of thermal insulation on the transient temperature distribution of a robotic arm and hand moving in a nonhomogeneous temperature field is presented. A finite-difference scheme is used to find the transient temperature distribution for a composite two-dimensional cylindrical rod moving periodically into and out of a hot environment. One part of the rod simulates the robotic hand with its insulation and the other part simulates the robotic arm. The heat transfer to the surface is by convection and radiation. Due to the periodic motion, the heat transfer coefficient has spatial and time dependence and the environmental condition is time dependent. The effect of thickness of insulation covering the robotic hand on the temperature distribution in the robotic arm and hand for fast, intermediate, and slow transients is studied and discussed.

Horizontal In-Tube Condensation Heat Transfer in a Low- to Negative-Quality Region

Abstract: Condensation heat transfer in a low- to negative-quality region was analyzed using a nonequilibrium heat transfer model of stratified flow. This analysis predicted that a condensing flow remains two-phase until the thermal equilibrium quality becomes negative due to condensate subcooling. This phenomenon at thermal equilibrium zero quality brings about a much higher heat transfer coefficient than the liquid single-phase flow value. To examine the validity of this analysis, an experiment was conducted using Refrigerants R113 and R11 in the negative-quality region. Data obtained and other existing data for Refrigerants R11 and R12 supported the analysis.

Modeling Varying-Heat-Transfer-Area Batch Processe

Abstract: Analytical equations are developed for heating and cooling of liquid batches, where the volume changes with time. Both reacting and nonreacting batches, as well as isothermal and nonisothermal exchange media, are considered. The operations of evaporation and exothermic chemical reaction are discussed as typical examples of batch heating and batch cooling. Previous effort at modeling varying-heat-transfer-area batch processes involved the use of a chart, and the calculations required graphical/numerical integration in the case of a nonisothermal exchange medium; here, analytical solutions are presented for the lime dependence of the batch volume and the heat transfer rate. Furthermore, the previously made assumption of the arithmetic mean for the temperature driving force is shown to be unnecessary, as the log-mean temperature difference leads to a simple, appropriate analysis.