Convective heat transfer to water near the critical region in a horizontal square duct

Numerical analysis has been carried out to investigate forced convective heat transfer to water near the critical region in a horizontal square duct. Near the critical point convective heat transfer in the duct is strongly coupled with large variation of thermophysical properties such as density and specific heat. Buoyancy force parameter has also severe variation with fluid temperature and pressure in the duct. There is flow acceleration along the horizontal duct resulted from fluid density decrease due to the heat transfer from the wall. Local heat transfer coefficient has large variation along the inner surface of the duct section and it depends on pressure. Nusselt number on the center of the bottom surface also has a peak where bulk fluid temperature is higher than the pseudocritical temperature and the peak decreases with the increase of pressure. Flow characteristics of velocity, temperature, and local heat transfer coefficient with water properties are presented and analyzed. Nusselt number distributions are also compared with other correlations for various pressures in the duct.     

Transient convective heat transfer to water near the critical region in a vertical tube

Numerical analysis has been carried out to investigate transient forced convective heat transfer to water near the critical region in developing flow through a vertical tube. With large variations of thermophysical properties such as density, specific heat, viscosity, and thermal conductivity near the thermodynamic critical point, heat transfer in the tube is strongly coupled with fluid flow. Buoyancy force parameter has also large variation with fluid temperature and pressure in the tube. Time dependent characteristics of fluid velocity, temperature, and heat transfer coefficient with water properties are presented and analyzed. Transient Nusselt and Stanton number distributions along the tube are also compared for various pressures in the tube. Because of heat transfer from the wall transition behavior from liquid-like phase to gas-like phase of heat transfer coefficient occurs when the fluid passes through pseudocritical temperature region in the tube. Turbulent viscosity ratio also has steep variation near the pseudocritical temperature close to the wall.     

A generalized multifluid optimal pressure for heat exchangers operating with supercritical fluid

Distinct thermophysical properties encountered in supercritical fluids operating near the critical point have made them strong candidates for working fluids in various engineering applications. Particularly due to the existence of heat capacity maxima near the critical point, heat transfer involving supercritical fluids and their employment in power generation systems have received special attention. In this paper, the existence of optimal operating pressures that maximize the global conductance of supercritical heat exchangers is demonstrated. Analysis of the behavior of the isobaric specific heat along the heat transfer process shows that optimal performance is achieved when the average isobaric specific heat is maximized. Consequently, optimal pressure maps can be created to assist heat exchanger design for various combinations of inlet temperatures and heat transfer rates. Furthermore, it is demonstrated that simple dimensionless groups can correlate—with a mean absolute error (MAE) of 0.0332—the optimal operating pressures of up to 122 different fluids. In addition, it is shown that the correlation is even stronger closer to the critical point and for separate classes of fluids, where MAE can be as low as 0.0103 for triatomic substances.     

Study on carbon dioxide gas cooler heat transfer process under supercritical pressures

In carbon dioxide transcritical air‐conditioning and heat pump systems, the high‐pressure‐side heat exchanger operating at supercritical pressures is usually called as gas cooler. The carbon dioxide gas cooler displays much difference from the traditional heat exchangers employing constant property fluids. The commonly used logarithmic mean temperature difference (LMTD) and effectiveness—heat transfer unit (ε‐NTU) fail for the gas cooler design calculation as the carbon dioxide properties change sharply near the critical or pseudo‐critical point in the heat transfer processes. The new effective heat transfer temperature difference expression for variable fluid property derived by the authors is verified by numeric simulation of the carbon dioxide gas cooler. Moreover, the available correlated models for the cooled carbon dioxide supercritical heat transfer are used to simulate the gas cooler. Detail analysis is made for the deviations among the different models, and for the distributions of local convective coefficient, heat flux, and local temperature of carbon dioxide along the flow path in the gas cooler. Copyright © 2002 John Wiley & Sons, Ltd.     

Numerical study of convective heat transfer to supercritical water in rectangular ducts

Numerical investigation has been performed to analyze forced convective heat transfer to supercritical water in horizontal rectangular ducts. Convective heat transfer near the critical region in the rectangular ducts is strongly influenced by large variations of thermodynamic and transport properties of supercritical fluid with gravity force, especially close to pseudocritical temperature. Fluid flow and heat transfer characteristics such as velocity, temperature, and local heat transfer coefficient with water properties distribution in the ducts are presented. Flow accelerates along the horizontal ducts because of decreased water density from heat transfer at the duct walls. Center of large flow recirculation in the duct section locates near the middle of vertical surface and additional secondary recirculation in clockwise direction appears with the increase of duct height. Local wall temperature severely varies along the inner surface of the duct section and its variation depends on aspect ratio of the duct. The heat transfer coefficient distributions along the ducts for various aspect ratios are compared with the proximity effect to the critical pressure.     

理想流体对流传热问题的理论解

研究理想流体受迫对流传热和自然对流传热问题的理论解。采用流体无垂直于壁面法线方向运动(即无穿透)的条件取代黏性流体在壁面无滑移条件，解决了流体在边界上有滑移时计算对流传热系数的困难，给出了理想流体与平壁受迫对流传热、理想流体与竖直壁面自然对流传热和理想流体在管内受迫对流传热的理论解。结果表明：理想流体的对流传热与黏性流体同样存在着热边界层。在外部流动的情况下，无论受迫对流传热还是自然对流传热，对流传热系数都与流体的导热系数、密度和比热三乘积的二分之一次方成正比。在管内受迫对流的情况下，当无因次长度大于0．05时，局部Nu和界面无因次温度分布都不再变化，对于恒热流边界条件，Nu等于8，截面无因次平均温度等于2；对于恒壁温边界条件，Nu等于5．782，截面无因次平均温度等于2．316。     

Effect of thermophysical properties models on the predicting of the convective heat transfer coefficient for low concentration nanofluid

The term of nanofluid refers to a solid–liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. In order to study the heat transfer behavior of the nanofluids, precise values of thermal and physical properties such as specific heat, viscosity and thermal conductivity of the nanofluids are required. There are a few well-known correlations for predicting the thermal and physical properties of nanofluids which are often cited by researchers to calculate the convective heat transfer behaviors of the nanofluids. Each researcher has used different models of the thermophysical properties in their works. This article aims to summarize the various models for predicting the thermophysical properties of nanofluids which have been commonly cited by a number of researchers and use them to calculate the experimental convective heat transfer coefficient of the nanofluid flowing in a double-tube counter flow heat exchanger. The effects of these models on the predicted value of the convective heat transfer of nanofluid with low nanoparticle concentration are discussed in detail.     

The Mean Temperature Difference Method for Micro Heat Exchanger Analysis Considering Property Variation

The influence of reducing dimensions to microscale on the performance parameters of parallel flow tubular micro heat exchanger is numerically analyzed considering the thermophysical property variation effect. Also, the improvement in convective heat transfer coefficient, at a given temperature, with the decrease in dimensions to microscale is investigated. In the micro heat exchanger, conventional method fails to give accurate performance analysis due to significant property variation. So, a new temperature difference, i.e. mean temperature difference correlation considering the property variation effect is derived. Operating conditions and geometric parameters are varied, keeping the inlet temperature of the both fluids and inlet velocity of the cold fluid fixed. The following performance parameters are evaluated: effectiveness, volumetric heat transfer coefficient, mean temperature difference, and log mean temperature difference. The reliability of the new mean temperature difference method is checked by calculating the percentage deviation between mean temperature difference and log mean temperature difference value for macroscale to microscale heat transfer. The results show that percentage deviation is maximum at microscale.     

Natural convection due to horizontal temperature and concentration gradients—1. Variable thermophysical property effects

Typically for single component fluids, the variation of thermophysical properties is negligible except in the presence of large temperature differences, and, therefore, has no appreciable effect on the heat transfer. In contradistinction, thermophysical properties can vary significantly due to concentration differences which affect the heat and mass transfer. This work examines the effects of thermophysical property variation on the heat and mass transfer in a cavity due to natural convection driven by combined thermal and solutal buoyancy forces. Results indicate that thermophysical property variations can appreciably influence heat and mass transfer and velocity distribution.     

Hydrodynamic and thermal performance prediction of functionalized MWNT-based water nanofluids under the laminar flow regime using the adaptive neuro-fuzzy inference system

ABSTRACT

Adaptive Neuro-Fuzzy Inference System (ANFIS) opens a new gateway in understanding the complex behaviors and phenomena for different fields such as heat transfer in nanoparticles. The ANFIS method is a shortcut to find a nonlinear relation between input and output and results in valid outcomes, especially in engineering phenomena, which is used here for determining the convective heat transfer coefficient. Using the ANFIS, the critical parameters in heat transfer including convective heat transfer coefficient and pressure drop are determined. To realize this issue, the thermophysical properties of non-covalently and covalently functionalized multiwalled carbon nanotubes-based water nanofluid were investigated experimentally. The results of simulation and their comparison with the experimental results showed an excellent evidence on the validity of the model, which can be expanded for other conditions. The proposed method of ANFIS modeling may be applied to the optimization of carbon-based nanostructure-based water nanofluid in a circular tube with constant heat flux.     

Thermal analysis of a direct-gain room with shape-stabilized PCM plates

The thermal performance of a south-facing direct-gain room with shape-stabilized phase change material (SSPCM) plates has been analysed using an enthalpy model. Effects of the following factors on room air temperature are investigated: the thermophysical properties of the SSPCM (melting temperature, heat of fusion and thermal conductivity), inner surface convective heat transfer coefficient, location and thickness of the SSPCM plate, wall structure (external thermal insulation and wallboard material) etc. The results show that: (1) for the present conditions, the optimal melting temperature is about 20 °C and the heat of fusion should not be less than 90 kJ kg⁻¹; (2) it is the inner surface convection, rather than the internal conduction resistance of SSPCM, that limits the latent thermal storage; (3) the effect of PCM plates located at the inner surface of interior wall is superior to that of exterior wall (the south wall); (4) external thermal insulation of the exterior wall obviously influences the operating effect and period of the SSPCM plates and the indoor temperature in winter; (5) the SSPCM plates create a heavyweight response to lightweight constructions with an increase of the minimum room temperature at night by up to 3 °C for the case studied; (6) the SSPCM plates really absorb and store the solar energy during the daytime and discharge it later and improve the indoor thermal comfort degree at nighttime.     

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow is numerically studied in order to improve the understanding of the complex heat transfer and optimum design of the combustor. The heat transfer performance of a porous media combustor strongly depends on the thermophysical properties of the porous material. In order to explore how the material properties influence reciprocating superadiabatic combustion of premixed gases in porous media (short for RSCP), a two‐dimensional mathematical model of a simplified RSCP combustor is developed based on the hypothesis of local thermal non‐equilibrium between the solid and the gas phases by solving separate energy equations for these two phases. The porous media is assumed to emit, absorb, and isotropically scatter radiation. The finite‐volume method is used for computing radiation heat transfer processes. The flow and temperature fields are calculated by solving the mass, moment, gas and solid energy, and species conservation equations with a finite difference/control volume approach. Since the mass fraction conservation equations are stiff, an operator splitting method is used to solve them. The results show that the volumetric convective heat transfer coefficient and extinction coefficient of the porous media obviously affect the temperature distributions of the combustion chamber and burning speed of the gases, but thermal conductivity does not have an obvious effect. It indicates that convective heat transfer and heat radiation are the dominating ways of heat transfer, while heat conduction is a little less important. The specific heat of the porous media also has a remarkable impact on temperature distribution of gases and heat release rate. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 336–350, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20120     

Flow boiling heat transfer characteristics of wide-boiling mixtures

Heat transfer coefficients in horizontal flow boiling of the wide-boiling binary mixture of C₂F₆/C₂H₂F₄ and of the ternary mixture C₂F₆/SF₆/C₂H₂F₄ are measured. The experiments are carried out with the condition of uniform inner wall temperature at the tube perimeter. Results of the two boiling regions—forced convective evaporation and nucleate boiling—are obtained. In the forced convective evaporation region the results are predicted best using a pure fluid correlation which takes mixture properties into account. In the nucleate flow boiling region a degradation of the heat transfer coefficients occurs. This degradation has to be predicted with a suitable mass transfer controlled model, based on the ideal heat transfer coefficient, provided that a certain heat flux limit is not exceeded. In the region of high heat fluxes an “effective” heat transfer coefficient, based on local total vaporization, is more suitable than the previously cited ideal heat transfer coefficient.     

Heat Transfer and Pressure Drop Characteristics of Dilute Alumina–Water Nanofluids in a Pipe at Different Power Inputs

This work addresses the effect of temperature on the thermophysical properties (i.e., density, viscosity, thermal conductivity, and specific heat capacity) of alumina–water nanofluid over a wide temperature range (25°C–75°C). Low concentrations (0–0.5% v/v) of alumina nanoparticles (40 nm size) in distilled water were used in this study. The pressure drop and the effective heat transfer coefficient of nanofluids were also estimated for different power inputs and at different flow rates corresponding to Reynolds numbers in the range of 1500–6000. The trends in variation of thermophysical properties of nanofluids with temperature were similar to that of water, owing to their low concentrations. However, the density, viscosity, and thermal conductivity of nanofluids increased, while the specific heat capacity decreased with increasing the nanoparticle concentration. The convective heat transfer coefficient of the nanofluid and the pressure drop along the test section increased with increasing the particle concentration and flow rate of nanofluid. Results showed that the heat transfer coefficient increases, while the pressure drop decreases slightly with increasing the power input. This is because of the fact that increasing power input to heater increases the bulk mean temperature of nanofluids, resulting in a decreased viscosity. The prepared nanofluids were found to be more effective under turbulent flow than in transition flow.     

Numerical analysis for forced convection over a flat plate in power law fluids

The analysis of forced convective heat transfer of power-law non-Newtonian fluids from a flat plate is studied. The cases of prescribed surface temperature and prescribed wall heat flux are considered. The results of local heat transfer are obtained by using implicit finite difference scheme to solve the nonsimilarity solution. It was found that local similarity solution is given the low accuracy results for the study of boundary layer heat transfer of power-law fluids.     

Thermal and flow behavior of ice slurries in a vertical rectangular channel—Part II. Forced convective melting heat transfer

The convective heat transfer characteristics of ice slurries flowing vertically upward in a rectangular channel have been experimentally investigated. At steady state, the local heat transfer coefficients were obtained during convective melting, and the effects of ice fraction, Reynolds number and wall heat flux were determined. To gain more insights into the flow properties, local measurements of the axial mixture velocity, temperature and ice fraction distributions were also made near a heated wall. Four main factors were shown to enhance the convective heat transfer coefficient of ice slurries relative to single-phase flow: (1) the ice fraction, (2) thermally and/or hydrodynamically developing flow conditions, (3) mixed-convection and (4) non-Newtonian effects experienced at Re < 4000. Two new and simple heat transfer correlations are also proposed for the practical design of compact heat exchangers involving phase change ice slurries.     

应用于日光温室墙体的相变材料热物性优化研究

建立日光温室计算传热模型,以室内空气温度和墙体内表面温度为指标,通过实验方法验证了所建立的传热模型准确性,最后分析相变材料相变温度、相变焓、导热系数、密度等热物性对室内最低温度和相变蓄热率的影响规律,确定被动式相变蓄热墙体和主-被动式相变蓄热墙体的最佳相变材料热物性,阐明了实际应用时相变材料选择原则。研究结果表明,所建立的日光温室传热模型具有较高准确性,可用于日光温室墙体相变材料热物性优化;主-被动式相变蓄热墙体最佳相变材料的相变温度为27 ℃,相变焓为200 kJ/kg,导热系数为0.35 W/（m·K）,密度为440 kg/m³,被动式相变蓄热墙体最佳相变材料的相变温度为26 ℃,相变焓为200 kJ/kg,导热系数为0.35 W/（m·K）,密度为792 kg/m³;最佳相变材料热物性应用时,2种墙体室内最低温度均可达到15.0 ℃,但是被动式相变蓄热墙体的相变蓄热率较主-被动式相变蓄热墙体减小29.5%。本研究可为相变材料在日光温室的高效利用提供参考。     

Local Convective Heat Transfer from a Horizontal Tube in an Acoustic Cavitation Field

An experiment has been carried out to investigate the local convective heat transfer from a horizontal circular copper tube in an acoustic cavitation field. The effects of acoustical parameters (including sound source intensity, the vibrator location and sound distance), fluid temperature and thermophysical properties of working fluid on heat transfer enhancement were studied, as well as the variation of heat transfer rate with sound source intensity at constant heat flux. Results show that convection heat transfer was remarkably enhanced due to disturbance and impingement by cavitation bubble. Among these cases tested, the maximum augmentation ratio of 3.95 was reached for acetone with a cluster of cavitation bubbles impinging perpendicularly on the tube surface.     

Comparative review of turbulent heat transfer of nanofluids

Three essential aspects of the turbulent-flow, convective heat transfer of nanofluids relevant to their applications are comparatively reviewed in detail based on both theoretical analyses and experimental data. These aspects are: (a) selection – the comparison criteria of the thermophysical property-related heat transfer performance of nanofluids and their base fluids, (b) design – the predictions of the heat transfer coefficients of nanofluids based on homogeneous fluid models by using nanofluid effective thermophysical properties, and (c) effectiveness – the enhancements of the heat transfer coefficients of nanofluids over their base fluids. This review, including research from the inception of nanofluids to date, quantifies the accuracy of bases for future nanofluid evaluation.     

Heat transfer of a non-Newtonian fluid (Carbopol aqueous solution) in transitional pipe flow

An experimental study of the forced convection heat transfer for non-Newtonian fluid flow in a pipe is presented. We focus particularly on the transitional regime. A wall boundary heating condition of heat flux is imposed. The non-Newtonian fluid used is Carbopol (polyacrylic acid) aqueous solutions. Detailed rheology as well as the variation of the rheological parameters with temperature are reported. Newtonian and shear thinning fluids are also tested for comparative purposes. The characterization of the flow and the thermal convection is made via the pressure drop and the wall temperature measurements over a range of Reynolds number from laminar to turbulent regime. Our measurements show that the non-Newtonian character stabilizes the flow, i.e., the critical Reynolds number to transitional flow increases with shear thinning and yield stress. The heat transfer coefficients are given and compared with heat transfer laws for different regime flows. Details when the heat transfer coefficient loses rapidly its local dependence on the Reynolds number are analyzed.