Experimental Determination of the Boiling Heat Transfer Coefficients of R-134a and R-417A on a Smooth Copper Tube

Flooded evaporators are widely used as compact cooling units to cool liquids. They consist of a shell-and-tube heat exchanger, with the fluid to cool flowing inside the tubes of the bundle and a refrigerant that evaporates over those tubes. Pool boiling on the external surface of the tubes is a very complex process, and therefore the boiling heat transfer coefficients (HTCs) should be determined experimentally. Copper and copper alloys tubes are commonly employed in such heat exchangers, due to their high thermal conductivity and relative low cost. On the other hand, refrigeration and air conditioning sectors are undergoing significant changes caused mainly by the necessity of replacing existing refrigerants with more environmentally friendly ones. This paper reports the experimental determination of the pool boiling HTCs of R-134a and R-417A blend on a smooth copper tube of 18.87 mm diameter, at two saturation temperatures of 10°C and 7°C. Although smooth tubes are not commonly used in shell-and-tube evaporators nowadays, it is a first approach to pool boiling of drop-in refrigerants. The experimental setup and data acquisition are described, the experimental procedure is explained, the data reduction methodology is detailed, and the results are presented and discussed.     

Effect of Flow Direction for the Heat Transfer Performance of Refrigerant R-410A Evaporation in a Plate Heat Exchanger

This article provides an experimental investigation of the effect of flow direction for refrigerant R-410A evaporated in a plate heat exchanger. Parallel-flow and counterflow arrangements with 2°C and 5°C exit superheat conditions were tested. The refrigerant entered the test section at a vapor quality of 0.24 and evaporated at a saturation temperature of 1.1°C. The experimental results were analyzed by the evaporation heat transfer coefficient and overall average heat transfer coefficient separately. The evaporation heat transfer coefficient in parallel-flow arrangement is higher than that in the case of counterflow arrangement. However, the average heat transfer coefficients are affected not only by the flow direction, but also by the exit superheat condition. The interaction of these two effects causes there to be almost no difference of the average heat transfer performance between these two flow arrangements for low exit superheat condition. While the refrigerant exit superheat is high, the overall heat transfer performance of the parallel-flow case is lower than that of the counterflow case.     

Experimental Study on R-410a Evaporation Heat Transfer Characteristics in Oblong Shell and Plate Heat Exchanger

The evaporation heat transfer experiments were conducted with an oblong shell and plate heat exchanger without oil in the refrigerant loop using R-410A, a mixture of 50 wt% R-32 and 50 wt% R-125 that exhibits azeotropic behavior. An experimental refrigerant loop has been established to measure the evaporation heat transfer coefficient h _r of R-410A in a vertical oblong shell and plate heat exchanger. Four vertical counter-flow channels were formed in the oblong shell and plate heat exchanger by four plates having a corrugated trapezoid shape of a 45° chevron angle. The upflow of the boiling R-410A in one channel receives heat from the hot downflow of water in the other channel. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature, and vapor quality of R-410A on the measured data were explored in detail. The results indicate that a rise in the refrigerant mass flux causes an increase in the h _r . Raising the imposed wall heat flux was found to slightly improve h _r . Finally, at a higher refrigerant saturation temperature, the h _r is found to be lower. Based on the present data, an empirical correlation of the evaporation heat transfer coefficient was proposed.     

Convective Heat Transfer Coefficients in Concentric Annuli

Abstract

The geometric shape of a passage's cross-section has an effect on its convective heat transfer capabilities. For concentric annuli, the diameter ratio of the annular space plays an important role. The purpose of this study was to determine to what extent research has been done on convective heat transfer in smooth concentric annuli and, if possible, to improve on or contribute to existing theories. It was found that although various correlations exist, they are not in good agreement. For this study, experiments were conducted with a wide range of annular diameter ratios. The Wilson plot method was used to develop a convective heat transfer correlation for annular diameter ratios of 1.7 to 3.2. For Reynolds numbers (based on the hydraulic diameter), in the range of 4000 to 30000, the deduced correlation predicted Nusselt numbers accurately within 3% of experimental values.     

分离式热管蒸发段传热特性的研究

在研究分离式热管蒸发段流动特性的基础上,对其传热特性进行了系统的试验研究和理论分析,首先分析了核态沸腾传热区及飞溅降膜传热区的换热原理;根据数据回归整理了核态沸腾传热区和飞溅降膜传热区的换热系数无量纲准则关系式,与试验数据吻合较好;并将它们与其它关系式进行了比较,得出了有用的结论,其结果为分离式热管的研究及工程应用提供了理论依据。     

Flow Boiling Heat Transfer of Refrigerant R-134a in Copper Microchannel Heat Sink

In this paper we present experimental data on heat transfer and pressure drop characteristics at flow boiling of refrigerant R-134a in a horizontal microchannel heat sink. The primary objective of this study was to experimentally establish how the local heat transfer coefficient and pressure drop correlate with the heat flux, mass flux, and vapor quality. The copper microchannel heat sink contains 21 microchannels with 335 × 930 μm² cross section. The microchannel plate and heating block were divided by the partition wall for the local heat flux measurements. Distribution of local heat transfer coefficients along the length and width of the microchannel plate was measured in the range of external heat fluxes from 50 to 500 kW/m²; the mass flux varied within 200–600 kg/m²-s, and pressure varied within 6–16 bar. The obvious impact of heat flux on the magnitude of heat transfer coefficient was observed. It showed that nucleate boiling is the dominant mechanism for heat transfer. A new model of flow boiling heat transfer, considering nucleate boiling suppression and liquid film evaporation, was proposed and verified experimentally in this paper.     

降膜蒸发过程的传热性能研究

对圆管和平壁的降膜蒸发过程的传热机理及影响因素进行了分析,并对已有的实验结果进行了比较;已有的实验研究主要是围绕圆管或平壁来进行,不利于找出最佳加热元件;提出在平壁表面加圆管型肋的复合加热元件,对其进行研究有利于找出最佳加热元件,对提高降膜蒸发传热效率极具实用价值。     

Discharge Coefficients and Heat Transfer for Axisymmetric Supersonic Nozzles

Computational fluid dynamics (CFD) analysis was used to compute effective nozzle discharge coefficients for subscale sharp-edged converging/diverging nozzles, with a variety of convergence half-angles, motor operating conditions, and two propellants with different ballistics. Convergence half-angles ranged from 10° to 90°. Analysis was conducted at total temperatures from 2,946 K (5303°R) to 3,346 K (6023°R) and total pressures ranging from 2.72 MPa (395 psia) to 20.68 MPa (3,000 psia). Area ratios (A e /A*) ranged from 7.43 to 9.39. Ratio of specific heats ( n ) ranged from 1.13 to 1.18. The maximum throat and exit Reynolds' numbers based on axial diameter ranged from 6.73 2 105 to 3.61 2 106 and 3.26 2 105 to 1.99 2 106, respectively. Present results of nozzle discharge coefficients are reported and correlated as a function of nozzle convergence half-angle ( / c ), area ratios (A e /A*), and pressure ratio (P o /P X ) for a constant divergence half-angle ( / d ) of 15°. Computed discharge coefficients ranged from 0.88 to 0.97. They are compared with theory and experimental data available in the literature. Available turbulence models with respect to grid refinements and heat transfer are discussed. Heat transfer is calculated from a modified Reynolds' analogy for laminar flow over a flat plate, the Dittus - Boelter correlation for fully developed turbulent pipe flow, and the Bartz correlation for nozzle flows, and the results are compared with available experimental data.     

超临界和超超临界汽轮机汽缸传热系数的研究

提出了汽轮机汽缸传热系数的计算方法。介绍了超，临界和超超临界压力汽轮机汽缸光滑内表面和安装镶片式汽封表面的对流换热表面传热系数的计算公式，安装整体车制式汽封的汽缸内表面、安装静叶的汽缸内表面和安装隔板的汽缸内表面的传热过程总传热系数的计算方法。采用圆筒壁与肋片传热等简化模型来计算汽封块、静叶和隔板的传热过程总传热系数。给出了某型号超，临界600MW汽轮机高压内缸内表面传热系数的计算结果。该方法考虑了不同运行工况下汽缸不同部位的传热过程，在超临界和超超临界压力汽轮机汽缸的温度场与热应力场的有限元法数值计算和寿命评定中，为确定传热边界条件提供了依据。     

Heat Transfer Coefficients for Turbulent Flow in Concentric Annular Ducts

On the basis of a large number of experimental data from the literature, correlations were developed for the heat transfer coefficient for turbulent flow in concentric annular ducts. A proven correlation for heat transfer in circular tubes was extended by factors that take into consideration the effect of the diameter ratio of the annulus and the different boundary conditions for heating or cooling.     

Heat Transfer Model with Two Heat Transfer Coefficients Along a M ultiperforated Plate-Application to Combustion Chamber Wall Cooling

Walls' cooling of aeronautic propeller combustion chamber is performed with the injection, through the combustion chamber wall, of a part of the air coming from compressors placed upstream. Measurements of the wall thermal fields are made by infrared thermography along the injection wall. This injection wall is pierced by 9 rows of 8 holes (α=90°) in staggered configuration(p/D=s/D=6). We propose a model using two heat transfer coefficients to represent the convective exchanges. The results are non-dimensioned and presented in comparison with the case without holes. The use of this model allows us to define 4 zones. Those 4 zones exist for the 5 blowing rates.     

The Challenge to Measure Single-phase Convective Heat Transfer Coefficients in Microchannels

ABSTRACT

The determination of local convective heat transfer coefficients in microfluidics is a very hard task. Due to the small dimensions of channels and walls, the use of conventional measurement techniques is only partially suitable in microfluidics. For this reason, a strong effort has been made during the last decades in order to propose innovative techniques which use internal (to microdevices) sensors of reduced dimensions and/or external conventional sensors. In this paper a review of the main experimental techniques proposed for the determination of the local near-wall fluid temperature, the local wall temperature, and the local fluid bulk temperature will be given by putting in evidence for each technique's positive and negative aspects as well as their actual limitations with the aim to stimulate and address the research on this topic in the near future. The problems and the limitations existing nowadays for the accurate measurements of the local thermal properties of a convective microflow demonstrate that for the analysis of microconvection experimental data have to be always integrated by a numerical modeling of the observed system.     

Single-Phase Convective Heat Transfer and Pressure Drop Coefficients in Concentric Annuli

Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers are known to have an effect on their convective heat transfer capabilities. Linear and nonlinear regression models exist for determining the heat transfer coefficients; however, these are complex and time-consuming, and require much experimental data in order to obtain accurate solutions. A large data set of experimental measurements on heat exchangers with annular diameter ratios of 0.483, 0.579, 0.593, and 0.712 with respective hydraulic diameters of 17.01 mm, 13.84 mm, 10.88 mm, and 7.71 mm was gathered. Mean Nusselt numbers were determined using the modified Wilson plot method, a nonlinear regression scheme, and the logarithmic mean temperature difference method. These three methods presented disagreements with existing correlations based on local wall temperatures. The local Nusselt numbers were determined using the logarithmic mean temperature difference method. Local wall temperature measurements were made using a novel method that minimized obstructions within the annulus. Friction factors were calculated directly from measured pressure drops across the annuli. Both heated and cooled horizontal annuli in fully turbulent flow with Reynolds numbers based on the hydraulic diameter varying from 10,000 to 45,000 with water as the working medium were investigated.     

Comparison of External Surface Heat Transfer Coefficients for Circular and Oval Tubes

Experimental measurements were carried out to compare the air-side heat transfer coefficients of three oval tubes (axis ratio 2, 3, and 4) with those of an equivalent circular tube (o. d. 18 mm). The tubes were tested as single tubes in a cross-flow of air. The range of the investigated Reynolds numbers Re _D was between 1000 and 11000. The effects of the area blockage and the free stream turbulence were taken into consideration in the evaluation of the thermal performance. The measurement results indicate that Nusselt numbers Nu _D for the tested circular and oval tubes are close at the lower range of the tested Reynolds numbers (Re _D < 4000) corresponding to an air velocity < 4 ms ^?1 in this work, which is the air velocity for most air conditioning applications. For Re _D > 4000, the Nu _D for the circular tube are higher than those for the oval tubes, and the Nu _D for the oval tubes decrease with the increase of the axis ratio.     

The Influence of Improved Physical Property Data on Calculated Heat Transfer Coefficients

Heat transfer coefficients calculated from accepted correlations, but using physical properties derived from different sources, are compared with experimentally determined local heat transfer coefficients. In all cases, consistent, accurate property data yield more accurate estimates of the heat transfer coefficients.     

Calculation of Two-Phase Convective Heat Transfer Coefficients of Cryogenic Nitrogen Flow in Plate-Fin Type Heat Exchangers

The two-phase convective heat transfer coefficients for nitrogen inside the flow path of plate-fin type heat exchangers operating at cryogenic temperatures are calculated using CFX Release 13.0. Using a homogeneous two-phase model, the governing equations are solved to find pressure, velocity, and enthalpy distributions for three types of fin geometries: plain, wavy, and serrated. The results are further processed to evaluate the wall shear stress and heat flux, which in turn yield the friction coefficients and convective heat transfer coefficients. The coefficients are presented as functions of system pressure, flow rate, and local quality. The results can be used for the design of plate-fin type exchangers with the same fin configurations and operating conditions as the calculation.     

Effect of Relative Humidity on the Prediction of Natural Convection Heat Transfer Coefficients

Results of an investigation into the sensitivity of natural convection heat transfer correlations with respect to relative humidity are presented. Given the relatively small values of natural convection heat transfer coefficients, small changes in the thermophysical properties can have a significant impact on the values predicted by theoretical/empirical correlations. In this study, the thermophysical properties are assumed to be those of a dry air and water vapor mixture. The mole fractions are determined as a function of relative humidity. Several widely used natural convection heat transfer correlations have been examined to determine the impact of varying the relative humidity on the predicted Nusselt number. The results show a general trend of an increasing Nusselt number with relative humidity. The results presented in this paper provide an engineering tool for obtaining accurate values of natural convection heat transfer coefficients for a moist air environment using only the thermophysical properties of dry air.     

Evaporation Heat Transfer Coefficient in a Capillary Pumped Loop and Loop Heat Pipe for Different Working Fluids

Capillary pumped loop (CPL) and loop heat pipe (LHP) are passive two-phase heat transport devices. They have been gaining importance as a part of the thermal control system of spacecraft. The evaporation heat transfer coefficient at the tooth–wick interface of an LHP or CPL has a significant impact on the evaporator temperature. It is also the main parameter in sizing of a CPL or LHP. Experimentally determined evaporation heat transfer coefficients from a three-port CPL with tubular axially grooved (TAG) evaporator and a TAG LHP with acetone, R-134A, and ammonia as working fluids are presented in this paper. The influences of working fluid, hydrodynamic blocks in the core, evaporator configuration (LHP or CPL), and adverse elevation (evaporator above condenser) on the heat transfer coefficient are presented.     

Estimating Heat Transfer Coefficients and Friction Factors in Non-Newtonian Flows between Parallel Plates

A simplified method, successfully tested previously for flow in circular pipes, is used in this work to estimate the friction factor and Nusselt number in fully-developed laminar flow between parallel plates of non-Newtonian fluids. Both constant wall temperature and constant wall heat flux cases are considered. The methodology was tested using several constitutive equations, including generalized Newtonian fluids, such as the Herschel–Bulkley, Bingham, Casson, and Carreau–Yasuda models, and also the simplified Phan-Thien–Tanner viscoelastic model. The error of the approximate methodology was found to be small, below 3.4%, except for the fluids with yield stress for which the maximum error increased to 8.4% for the cases analyzed, which cover a wide range of shear viscosity curves.     

流化床表面传热系数的直接数值模拟

采用数值试验方法对表面传热系数进行了直接数值模拟．在流化床表血传热系数模型中,流体相的运动和传热规律以Euler方法描述,对固体颗粒相运动和传热规律则以离散单元法(DEM)在颗粒层次上进行描述．利用该模型,对一个二维鼓泡流化床内瞬时和局部传热系数进行了模拟,得到了瞬态表面传热系数随流化速度的变化规律,以及局部传热系数随高度的变化规律．该规律与Ozkaynak等人的实验研究结果以及Syamlal等人采用颗粒相拟流体模型的数值模拟结果相一致,但与双流体数学模型相比,该模型所需主观假设较少且适用范围更广．