振动激励下的封闭通道流动传热特性实验研究

为了对振动激励下的自然对流换热规律进行深入的理论与试验分析,搭建电动振动传热试验平台与并行式数据采集系统,测量封闭通道在不同振动激励下的温度瞬态变化特性。研究发现:随着受迫振动频率的增加,振动激励可以将自然对流强化为混合对流;当频率超过50 Hz时,自然对流的影响可以忽略。但是由于受迫振动下的流动状态依然为层流,因而对于自然对流的最大强化程度约在100%。另外,通过对混合对流状态的理论分析,得到由于受迫振动引起的封闭换热通道内部强制对流换热试验关联式。     

Aiding and opposing mechanisms of mixed convection in a shear- and buoyancy-driven cavity

The present study was conducted to numerically investigate the transport mechanism of laminar combined convection in a shear- and buoyancy-driven cavity. The focus was on the interaction of the forced convection induced by the moving wall with the natural convection induced by the buoyancy. Two orientations of thermal boundary conditions at the cavity walls are considered in order to simulate the aiding and opposing buoyancy mechanisms. Velocity and temperature distributions of the flow are carried out through a stream function-vorticity transformation with a finite difference scheme. Parametric studies of the effect of the mixed convection parameter, Gr/Re², on the fluid flow and heat transfer have been performed. Calculations cover Re=100, Pr=0.71 and Gr/Re² the range of 0.01–100. Three different regimes are observed with increasing Forced convection (with Gr/Re² negligible natural convection), mixed convection (comparable forced and natural convection) and natural convection (with negligible forced convection). The code developed was carefully tested for the two limiting cases, the pure forced convection and the pure natural convection, for which experimental and numerical data are available.     

非均匀热流下水平管内混合对流大涡模拟研究

针对以槽式太阳能集热器为背景的高密度、高度非均匀热流下水平管内的混合对流换热问题,采用大涡模拟方法,研究了热流密度非均匀性对水平管内混合对流瞬态涡结构、脉动强度、湍流热通量及局部平均壁温的影响;揭示了非均匀热流下自然对流对管内湍流特性的影响规律;提出了适用于不同热边界条件下管内混合对流换热的强化措施。结果表明:均匀热流时,自然对流会抑制管顶部的湍流脉动,使流动层流化,造成传热能力局部恶化;非均匀热流时,随着自然对流的增强,近壁面速度脉动强度先减小后增大,二次流逐渐增强,换热能力逐渐提高,故管内换热能力受湍流脉动与二次流协同影响;在自然对流影响下,均匀加热时管顶部可采用针对层流的强化换热措施,非均匀加热时需着重提高管底部高热流区域的湍流脉动与涡强度。     

Combined forced and natural convection heat transfer for upward flow in a uniformly heated,vertical pipe

For predicting the fully developed upward flow in a uniformly heated, vertical pipe by taking account of the buoyancy force, the k-ε models of turbulence for low Reynolds number flows were adopted. The regime map for forced, mixed and natural convections as well as for laminar and turbulent flows was plotted from the numerical predictions. At the same time, experiments were carried out at Reynolds numbers of 3000 and 5000, with the Grashof number varying over a wide range, by using pressurized nitrogen gas as a test fluid. In agreement with the prediction, buoyancy-induced impairment of heat transfer was correctly measured in the mixed convection regime. Furthermore, from hot-wire measurements, complete laminarization was demonstrated in the mixed-convection region at a Reynolds number of 3000.     

Combined forced and natural convection heat transfer from a horizontal cylinder embedded in a porous medium

This paper reports the results of an experimental study of heat transfer by combined forced and natural convection from a horizontal cylinder embedded in a porous medium composed of randomly packed glass spheres saturated with water. The direction of the flow of water was horizontal and normal to the longitudinal axis of the cylinder. The diameter of the cylinder, D, was 11.45mm and the equivalent diameter of the glass spheres was 3.072mm. It is shown that the condition Gr_k/Re²_D ⩽ 0.5 represents a conservative criterion for segregating heat transfer data that are predominantly governed by forced convection from those in which natural convection effects are significant. A correlation hypothesis for convection heat transfer which is based upon four assumptions, primary among which is that the flow can be (conceptually) regarded as being composed of ‘coarse’ and ‘fine’ components, is presented. This hypothesis is shown to provide a basis for successfully correlating a set of experimental heat transfer data that extends from the Darcy regime into the turbulent regime and spans the intervening Forchheimer and transition regimes. It is suggested that the correlation procedure adopted here may yield useful results if applied to other geometries such as, for example, forced convection heat transfer in ducts packed with porous media.     

Contribution Analysis of Dimensionless Variables for Laminar and Turbulent Flow Convection Heat Transfer in a Horizontal Tube Using Artificial Neural Network

The artificial neural network (ANN) method has shown its superior predictive power compared to the conventional approaches in many studies. However, it has always been treated as a “black box” because it provides little explanation on the relative influence of the independent variables in the prediction process. In this study, the ANN method was used to develop empirical correlations for laminar and turbulent heat transfer in a horizontal tube under the uniform wall heat flux boundary condition and three inlet configurations (re-entrant, square-edged, and bell-mouth). The contribution analysis for the dimensionless variables was conducted using the index of contribution defined in this study. The relative importance of the independent variables appearing in the correlations was examined using the index of contribution based on the coefficient matrices of the ANN correlations. For the turbulent heat transfer data, the Reynolds and Prandtl numbers were observed as the most important parameters, and the length-to-diameter and bulk-to-wall viscosity ratios were found to be the least important parameters. The method was extended to analyze the more complicated forced and mixed convection data in developing laminar flow. The dimensionless parameters influencing the heat transfer in this region were the Rayleigh number and the Graetz number. The contribution analysis clearly showed that the Rayleigh number has a significant influence on the mixed convection heat transfer data, and the forced convection heat transfer data is more influenced by the Graetz number. The results of this study clearly indicated that the contribution analysis method can be used to provide correct physical insight into the influence of different variables or a combination of them on complicated heat transfer problems.     

Approximate solution to the natural convection heat transfer from a vertical plate

A new model for laminar natural convection heat transfer between an isothermal vertical plate to a power-law fluid is proposed. The difficulty caused by the fact that the momentum and energy equations are coupled is avoided by introducing an approximation based on the partial similarity between the velocity fields for forced and natural convections. The model proposed in this work agrees well with the available experimental data and correlations. Further, we have extended the analysis to turbulent natural convection heat transfer. The predicted turbulent heat transfer rates are in satisfactory agreement with the data and the correlations published in the literature.     

Transition from natural to mixed convection for steam–gas flow condensing along a vertical plate

An analysis method based on two-phase boundary layer analysis has been developed to study the effects of superimposed forced convection on natural convection steam–gas flow condensing along a vertical plate. The mechanism by which superimposed forced convection enhances heat transfer is evaluated: the bulk flow blows away non-condensable gases accumulating near the interface, resulting in an elevated condensation driving force. Further, this bulk flow blowing capability may be characterized by a conventional mass transfer driving potential. Results of the new model are shown to be consistent with experimental data. Finally, a simple criterion was developed to identify transition to mixed convection from natural convection steam–gas flow.     

Exact solution of the transient forced convection energy equation for timewise variation of inlet temperature

An exact solution to the equation of transient forced convection for time varying inlet temperature with a general, space dependent boundary condition of an incompressible laminar forced convection heat transfer with fully developed flow between two parallel plates is given. The finite integral transform technique has been used as the method of analysis. Analytical results for laminar and turbulent flow are presented. The results are confirmed experimentally by the frequency response method.     

Turbulent Free Convection from a Vertical,Isothermal Plate

Abstract

The temperature and velocity fields adjacent to an isothermal, vertical plate have been calculated by finite-difference methods using the time-averaged equations of conservation. Effective diffusivities for turbulent transfer were computed from differential balances for the turbulent kinetic energy and the rate of dissipation of turbulent energy, using the empirical coefficients previously evaluated for forced convection together with an additional Prandtl-number-dependent coefficient associated with the buoyant production of turbulent kinetic energy. Although the turbulent boundary layer almost approaches an asymptotic condition, the slight development requires a two-dimensional solution proceeding from the leading edge through the laminar and turbulent regimes. It was necessary to trigger the transition from laminar to turbulent motion, but the ultimate turbulent behavior was found to be independent of the point of triggering. The computed velocity fields, temperature fields, and rates of heat transfer are in agreement with prior experimental data for air within their uncertainty. Excellent agreement was also obtained with the data for water, and fair agreement with the data for spindle oil.     

Deteriorated turbulent heat transfer (DTHT) of gas up-flow in a circular tube: Heat transfer correlations

This paper presents comparisons of recent experimental data, presented in a companion paper, to the existing correlations for heat transfer in various gas flow regimes and development of more accurate new correlations. The existing correlation of Celeta et al. showed the best agreement with the data in the turbulent heat transfer regime, while most of the existing correlations for laminar heat transfer showed unsuccessful predictions. Three new sets of correlations, each covering the mixed convection laminar, normal turbulent and deteriorated turbulent heat transfer (DTHT) regimes for heated gas up-flow, have been developed to agree better with the data in each regime.     

Turbulent heat and momentum transfer of combined forced and natural convection along a vertical flat plate—aiding flow

An aiding flow of turbulent, combined convection along a vertical flat plate is investigated experimentally in the range of high values of Rax¹ and Rex numbers. Local Nusselt numbers in the combined convection region are found to decrease as much as 25% than those for the pure forced and natural convection. It is revealed from the measurements of velocity and temperature that the reductions in heat transfer are mainly caused by the turbulent suppression. Turbulent transport mechanisms within the combined convective boundary layers are also discussed from the visualized data of the flow and temperature fields over the test plate.     

Inverse determination of building heating profiles from the knowledge of measurements within the turbulent slot-vented enclosure

Slot ventilated enclosure flows have been simulated, respectively in displacement ventilation and mixed ventilation covering from the forced convection dominated flow to the natural convection dominated flow. Direct convection simulation together with the turbulent streamlines and turbulent heatlines demonstrate that the enclosure flow pattern, indoor thermal level and heat transfer potential will depend on the interactions of external forced flow and thermal buoyancy driven flows, i.e., Reynolds number and Grashof number. In subsequent inverse convection modeling, the inverse determination of enclosure wall heat flux profiles was conducted by the use of adjoint methodology, in which the direct, sensitivity and adjoint problems are formulated and solved by finite volume method. The effects of the supplying air flow rate, thermal source strength, ventilation mode, flux functional forms, and the measurement errors on the accuracy of inverse turbulent convection estimation have been investigated. The inverse solutions of turbulent convections are of low level accuracy as the flow becomes thermal-driven turbulent flows, and they deteriorate as the noise levels increase. This work is of fundamental importance for the room air flow design and measurements involving the turbulent thermal convections.     

An experimental study of thermal performance of shell-and-coil heat exchangers

In the present study an experimental investigation of the mixed convection heat transfer in a coil-in-shell heat exchanger is reported for various Reynolds and Rayleigh numbers, various tube-to-coil diameter ratios and dimensionless coil pitch. The purpose of this article is to assess the influence of the tube diameter, coil pitch, shell-side and tube-side mass flow rate over the performance coefficient and modified effectiveness of vertical helical coiled tube heat exchangers. The calculations have been performed for the steady-state and the experiments were conducted for both laminar and turbulent flow inside coil. It was found that the mass flow rate of tube-side to shell-side ratio was effective on the axial temperature profiles of heat exchanger. The results also indicate that the ? − NTU relation of the mixed convection heat exchangers was the same as that of a pure counter-flow heat exchanger.     

Forced-convective internal cooling of a horizontal equilateral-triangle cross-sectioned duct

An experimental determination of the steady-state forced convection from the internal surfaces of a horizontal, uniformly heated electrically, equilateral-triangle cross-sectioned duct with sharp corners has been undertaken. The average Nusselt number _D, using the hydraulic diameter as the characteristic physical dimension, is a constant if the flow in the duct is laminar, but a function of the air's Reynolds number, when the flow is turbulent. Non-dimensional correlations, which can be used for predicting the value of the steady-state rate of convective heat transfer from such a triangular duct into the airflow, have been deduced, viz. _D = 3·25 for laminar flows, i.e. Re_D < 1450; and _D = 0·012 Re_D^0·83 for turbulent flows, i.e. Re_D ≥ 1450.     

Convection coefficient equations for forced air flow over flat surfaces

In this paper, various comparisons among well-known equations of the convection heat transfer coefficient for forced air flow over flat surfaces and particularly over flat plate solar collectors, with the aim at arriving at a consensus on which of such equations is more accurate are carried out. Through the application of basic principles, various accuracies, inaccuracies and validations of the considered equations have been found and shown, and a consensus reached. Such consensual equation, which comes from the boundary layer theory and takes into account the determining laminar and turbulent flows as well as the wind direction and the decay of the convection coefficient over the surface, also showed close agreement with different experimental works and tends to represent more accurately the actual heat transfer from/to any flat surface submitted to forced convection.     

Heat transfer experiments for low flow of water in rod bundles

Heat transfer experiments are conducted for fully developed forced and natural flows of water through triangularly arrayed, seven uniformly heated rod bundles with P/D = 1.25, 1.38 and 1.5. For forced circulation experiments. Re ranges from 80 to 50000 and Pr from 3 to 8.5; while in natural circulation. Re varies from 260 to 2000, and Ra, from 8 × 10⁶ to 2.5 × 10⁸. The forced flow data fall into two basic flow regimes : turbulent and laminar flow. At the transition between these regimes, Re_T, which varies from 2200 for P/D = 1.25 to 5500 forP/D = 1.5, increases linearly with P/D. The turbulent heat transfer data is in good agreement (±15%) with Weisman's correlation, designed for fully developed turbulent flow in rod bundles at Re > 25 000. However, the laminar flow data shows the dependency of Nu on Re to be weaker than for turbulent flow. Natural circulation data indicates that rod spacing insignificantly affects heat transfer; for P/D = 1.38 and 1.5, Nu is correlated as Nu = 0.272.Ra_0.25^q.     

Experimental study of mixed convection and pressure drop in helically dimpled tubes for laminar and transition flow

This paper presents the experimental results carried out in dimpled tubes for laminar and transition flows and completes a previous work of the authors focused on the turbulent region. It was observed that laminar flow heat transfer through horizontal dimpled tubes is produced in mixed convection, where Nusselt number depends on both the natural convection and the entry region. Employing water and ethylene glycol as test fluids, the following flow range was covered: x^*=10⁻⁴–10⁻² and Ra=10⁶–10⁸.

The experimental results of isothermal pressure drop for laminar flow showed dimpled tube friction factors between 10% and 30% higher than the smooth tube ones. Moreover, it was perceived that roughness accelerates transition to critical Reynolds numbers down to 1400. Correlations for the laminar friction factor f=f(Re,h/d) and for the critical Reynolds Re_crit=Re_crit(h/d) are proposed. The hydraulic behaviour of dimpled tubes was found to depend mainly on dimple height.

In mixed convection, high temperature differences in the cross section were measured and therefore heat transfer was evaluated by a circumferentially averaged Nusselt number. Experimenal correlations for the local and the fully developed Nusselt numbers and are given. Results showed that at low Rayleigh numbers, heat transfer is similar to the smooth tube one whereas at high Rayleigh, enhancement produced by dimpled tubes can be up to 30%.     

On heat transfer in external natural convection flows using two nanofluids

In the present work, a theoretical model based on the integral formalism approach for both laminar and turbulent external natural convection is extended to nanofluids. By using empirical models based on experimental data for computing viscosity and thermal conductivity of water–alumina and water–CuO suspensions, a close attention is first focused on the influence due to increasing the volume fraction of nanoparticles on the heat transfer and then to the transition threshold between laminar and turbulent regimes. The heat transfer is shown to strongly depend on the flow regime and on particle volume fraction. A clear degradation of heat transfer is observed using nanofluids while compared to that of the base-fluid. Moreover, the fact of increasing the particle volume fraction tends to delay the occurrence of the flow transition to turbulence.     

Conjugate Mixed Convection Heat Transfer in Plane Laminar Wall Jet Flow

A numerical investigation of heat transfer from a uniformly heated slab of finite thickness by plane laminar wall jet flow under combined forced and natural convection, i.e., mixed convection, is presented. The problem has been solved for two classical cases such as Pr ? 1 and Pr ? 1. The effects of the Grashof number (Gr), Reynolds number (Re), Prandtl number (Pr), and thermal conductivity ratio (R_k) between the slab and fluid medium are investigated on the heat transfer characteristics, i.e., local Nusselt number, interface temperature, and average Nusselt number.