Effect of fluid thermal properties on the heat transfer characteristics in a double-pipe helical heat exchanger

Heat transfer characteristics of a double-pipe helical heat exchanger were numerically studied to determine the effect of fluid thermal properties on the heat transfer. Two studies were performed; the first with three different Prandtl numbers (7.0, 12.8, and 70.3) and the second with thermally dependent thermal conductivities. Thermal conductivities of the fluid were based on a linear relationship with the fluid temperature. Six different fluid dependencies were modeled. Both parallel flow and counterflow configurations were used for the second study.Results from the first study showed that the inner Nusselt number was dependent on the Prandtl number, with a greater dependency at lower Dean numbers; this was attributed to changing hydrodynamic and thermal entry lengths. Nusselt number correlations based on the Prandtl number and a modified Dean number are presented for the heat transfer in the annulus. Results from the second part of the study showed that the Nusselt number correlated better using a modified Dean number. The counterflow configuration had higher heat transfer rates than the parallel flow, but the ratio of these differences was not different when comparing thermally dependent properties and thermally independent properties.     

Characterization of the effect of Froude number on surface waves and heat transfer in inclined turbulent open channel water flows

Interfacial heat transport in open channel turbulent flows is strongly dependent on surface waves that can appear as a result of the interaction of bulk turbulence with the free surface. The paper describes wave/heat transfer phenomena in inclined turbulent open surface water flows. The experiments were conducted in a regime of transition from “weak” to “strong” turbulence, in which the stabilizing influences of gravity and surface tension are relatively small against the disturbing effects of turbulence. A key role of the Froude number, Fr_⊥, built through the surface-normal component of g has been revealed. As Fr_⊥ grows, the wave amplitude grows, and the frequency spectrum shifts towards shorter waves. These changes lead to a heat transfer improvement, enough to double the heat transfer coefficient. The experimental data have been compared with calculations based on a “K-ε” model. As a result, the range of applicability of the standard model has been established as Fr_⊥<2000. The turbulent Prandtl number has been evaluated for Fr_⊥<700.     

Effects of pressure work and radiation on natural convection flow around a sphere with heat generation

The effects of pressure work and radiation on natural convection flow around a sphere in presence of heat generation have been investigated in this paper. The governing equations are transformed into dimensionless non-similar equations by using set of suitable transformations and solved numerically by the finite difference method along with Newton's linearization approximation. Attention has been focused on the evaluation of shear stress in terms of local skin friction and rate of heat transfer in terms of local Nusselt number, velocity as well as temperature profiles. Numerical results have been shown graphically and also in tabular form for some selected values of parameter set consisting of heat generation parameter Q, radiation parameter Rd, pressure work parameter Ge and the Prandtl number Pr.     

Convection heat and mass transfer in a hydromagnetic flow of a second grade fluid in the presence of thermal radiation and thermal diffusion

The convection heat and mass transfer in a hydromagnetic flow of a second grade fluid past a semi-infinite stretching sheet in the presence of thermal radiation and thermal diffusion are considered. The governing coupled non-linear partial differential equations describing the flow problem are transformed into non-linear ordinary differential equations by method of similarity transformation. The resulting similarity equations are solved numerically using Runge-Kutta shooting method. The results are presented as velocity, temperature and concentration fields for different values of parameters entering into the problem. The skin friction, rate of heat transfer and mass transfer are presented numerically in tabular form. In addition, the results obtained showed that these parameters have significant influence on the flow, heat and mass transfer.     

Influence of thermal radiation on the boundary layer flow due to an exponentially stretching sheet

The effect of radiation on the boundary layer flow and heat transfer of a viscous fluid over an exponentially stretching sheet is studied. The homotopy analysis method (HAM) is employed to determine the convergent series expressions of velocity and temperature. The physical interpretation to these expressions is assigned through graphs. It is found that the effects of Prandtl and radiation numbers on the temperature are opposite.     

Interaction between turbulence and thermal radiation in combined heat transfer channel flows

The aim of the present work is to examine the effects of interaction between turbulence and thermal radiation on the fully developed turbulent channel flow with variable properties in the presence of combined mixed convection‐radiation heat transfer. The vertical and horizontal channels under study are formed by differentially heated flat parallel plates. Large eddy simulation and the low Mach number approach are used to solve the governing equations. Also, the radiative transfer equation is solved using the method. The results are achieved by developing a solver in an open‐source computational fluid dynamics toolbox. The main focus is to find out whether neglecting turbulence‐radiation interaction (TRI) is a valid assumption for such flows under consideration. The present results show that, in both configurations, the maximum values of emission TRI and incident TRI are 2% and 3%, respectively. These results are consistent with the previous findings suggesting that in the nonreactive flows, these two terms are negligible.     

Effects of thermal property variations on the liquid flow and heat transfer in microchannel heat sinks

Three-dimensional conjugate numerical simulations using the inlet, average and variable thermal properties respectively were performed for the laminar water flow and heat transfer in rectangular microchannels with D_h of 0.333 mm at Re of 101–1775. Both average and variable properties are adopted in data reduction. The calculated local and average characteristics of flow and heat transfer are compared among different methods, and with the experiments, correlations and simplified theoretical solution data from published literatures. Compared with the inlet property method, both average and variable property methods have significantly lower f_app, but higher convective heat transfer coefficient h_z and Nu_z. Compared with the average property method, the variable property method has higher f_appRe_ave and lower h_z at the beginning, but lower f_appRe_ave and higher h_z at the later section of the channel. The calculated Nu_ave agree well with the Sieder-Tate correlation and the recently reported experiment, validating the traditional macroscale theory in predicting the flow and heat transfer characteristics in the dimension and Re range of the present work.     

Numerical study of PEMFC heat and mass transfer characteristics based on roughness interface thermal resistance model

The thermal contact resistance (TCR) is the main component of proton exchange membrane fuel cell (PEMFC) thermal resistance due to the existence of surface roughness between the components of PEMFC, and the influence of TCR is often ignored in traditional three dimensional PEMFC simulations. In this paper, the heat and mass transfer characteristics including polarization curve, power density curve, temperature distribution, membrane water content distribution, membrane current density are studied under different component surface roughness conditions, and finally the effect of each TCR on the PEMFC performance is studied. It is found that under the same operating conditions, the TCR makes the radial heat transfer of the PEMFC decrease, and the temperature of the membrane electrode and the temperature difference of each component of the PEMFC is higher than that of the model without TCR. When the surface roughness of components in the PEMFC equals 1 μm, 2 μm, 3 μm, the cell current density decreases by 6.56%, 12.46% and 17.17% respectively when the output cell voltage equals 0.3 V, and the cell power density decreases by 3.64%, 7.54%, 13.14% respectively when the cell current density equals 1.2 A·cm^?2. When the TCR between the CL and PEM equals 0.003 K·m²·W^?1, 0.005 K·m²·W^?1, 0.01 K·m²·W^?1, the cell current density is increased by 2.30%, 3.65%, 6.74% respectively under the condition that the output cell voltage equals 0.3 V, and the cell power density is increased by 1.24%, 1.85%, 3.10% respectively when the cell current density equals 1.2 A·cm^?2. The results show that the numerical simulation of PEMFC cannot ignore the effect of TCR.     

Investigation of heat transfer characteristics of hydrogen combustion process in cylindrical combustors from microscale to mesoscale

In the field of micro and mesoscale combustion, the feature of flame-wall thermal coupling is of great significance because of its small scale nature. Thus, this work provides a comprehensive heat transfer analysis in cylindrical combustors from the perspective of numerical simulation. The combustor has a fixed length-to-diameter aspect ratio of 10, and the channel diameter is scaling up from 1 mm to 11 mm to explore the influence of chamber dimension on heat transfer and flame structure. The distribution of convective and radiative heat flux on inner surface, contribution of thermal radiation are given. Moreover, the role of radiation in flame structure is analyzed, and the convective and radiative heat losses are quantitatively analyzed. We find that radiative heat flux is smaller compared to convective heat flux, and the proportion of radiative heat flux becomes larger with an increasing diameter. Thermal radiation does not change the flame structure when the diameter is less than 3 mm. When the diameter is greater than 5 mm, thermal radiation changes the location of flame front. The heat loss becomes larger at a smaller diameter, and heat loss ratio can reach approximately 73.6% in the combustor with diameter of 1 mm.     

Momentum boundary layer and its influence on the convective heat transfer in porous media

Convective heat transfer in a channel filled with a porous medium has been analyzed in this paper. The flow field is analyzed considering both the inertia and solid boundary effects and the thickness of the momentum boundary layer is found as a function of the Darcy and the Reynolds number. The two-equation model is applied for the heat transfer analysis and theoretical solutions are obtained for both fluid and solid phase temperature fields. The Nusselt number is obtained in terms of the relevant physical parameters, such as the Biot number for the internal heat exchange, the ratio of effective conductivities between the fluid and solid phases, and the thickness of the momentum boundary layer. The results indicate that the influence of the velocity profile is characterized within two regimes according to the two parameters, the Biot number and the conductivity ratio between the phases. The decrease in the heat transfer due to the momentum boundary layer is 15% at most within a practical range of the pertinent parameters.     

Capacitive effect on the heat transfer through building glazing systems

In recent years, several intensive studies have been carried out in order to reduce the energy consumption of buildings. One solution lies on whole building energy simulation that permits to enable the heat (and moisture) transfer through the building envelope and, consequently, is a way to understand how to improve the building performance. This article aims to analyze the modeling level needed to successfully evaluate the heat transfer through glazing parts of windows in such whole-building simulations as it is well-known that windows are the thermally weakest elements of the building envelope.     

The inaccuracy of heat transfer characteristics of insulated and non-insulated circular duct while neglecting the influence of heat radiation

The non-insulated and insulated ducts are commonly applied in the industries and various buildings, because the heat radiation equation contains the 4th order exponential of temperature which is very complicate in calculations. Most heat transfer experts recognized from their own experiences that the heat radiation effect can be ignored due to the small temperature difference between insulated and non-insulated surface and surroundings. This paper studies in detail to check the inaccuracies of heat transfer characteristics non-insulated and insulated duct by comparing the results between considering and neglecting heat radiation effect. It is found that neglecting the heat radiation effect is likely to produce large errors of non-insulated and thin-insulated ducts in situations of ambient air with low external convection heat coefficients and larger surface emissivity, especially while the ambient air temperature is different from that of surroundings and greater internal fluid convection coefficients. It is also found in this paper that using greater duct surface emissivity can greatly improve the heat exchanger effect and using smaller insulated surface emissivity can obtain better insulation.     

A study on entropy generation and heat transfer in a magnetohydrodynamic flow of a couple-stress fluid through a thermal nonequilibrium vertical porous channel

The effect of local thermal nonequilibrium (LTNE) on the entropy generation and heat transfer characteristics in the magnetohydrodynamic flow of a couple-stress fluid through a high-porosity vertical channel is studied numerically using the higher-order Galerkin technique. The Boussinesq approximation is assumed to be valid and the porous medium is considered to be isotropic and homogeneous. Two energy equations are considered one each for solid and fluid phases. The term involving the heat transfer coefficient in both equations renders them mutually coupled. Thermal radiation and an internal heat source are considered only in the fluid phase. The influence of inverse Darcy number, Hartmann number, couple-stress fluid parameter, Grashof number, thermal radiation parameter, and interphase heat transfer coefficient on velocity and temperature profiles is depicted graphically and discussed. The entropy generation, friction factor, and Nusselt number are determined, and outcomes are presented via plots. The effect of LTNE on the temperature profile is found to cease when the value of the interphase heat transfer coefficient is high, and in this case, we get the temperature profiles of fluid and solid phases are uniform. The physical significance of LTNE is discussed in detail for different parameters' values. It is found that heat transport and friction drag are maximum in the case of LTNE and minimum in the case of local thermal equilibrium. We observe that LTNE opposes the irreversibility of the system. The corresponding results of a fluid-saturated densely packed porous medium can be obtained as a limiting case of the current study.     

Study on the efficiency of effective thermal conductivities on melting characteristics of latent heat storage capsules

Improvement of the thermal conductivity of a phase change materials (PCM) is one effective technique to reduce phase change time in latent heat storage technology. Thermal conductivity is improved by saturating porous metals with phase change materials. The influence of effective thermal conductivity on melting time is studied by analyzing melting characteristics of a heat storage circular capsule in which porous metal saturated with PCM is inserted. Numerical and approximate analyses were made under conditions where there are uniform or non-uniform heat transfer coefficients around the cylindrical surface. Four PCMs (H₂O, octadecane, Li₂CO₃, NaCl) and three metals (copper, aluminum and carbon steel) were selected as specific materials. Porosities of the metals were restricted to be larger than 0.9 in order to keep high capacity of latent heat storage. Results show that considerable reduction in melting time was obtained, especially for low conductivity PCMs and for high heat transfer coefficient. Melting time obtained by approximate analysis agrees well with numerical analysis. A trial estimation of optimum porosity is made balancing the desirable conditions of high latent heat capacity and reduction of melting time. Optimum porosity decreases with increase in heat transfer coefficient.     

Correlation of the volumetric heat transfer coefficient for air-rock bed thermal storage system - considering humid air

Rock bed thermal storage with humid air green house or regenerative air cooling application are becoming widely investigated nowadays. The effect of the mass transfer in the circulated air has not received much attention. In this experimental study, the effect of the mass in the operated air is studied and new correlation for calculating the coefficient of volumetric heat transfer for humid air is presented.     

Heat transfer effect on the specific power availability of heat engines

The maximum possible specific power (specific power availability) that can be obtained from heat engines with a set of high temperature heat source and low temperature sink is analyzed. The heat engines considered in this paper include (1) externally and internally reversible, (2) externally irreversible and internally reversible, (3) externally reversible and internally irreversible and (4) externally and internally irreversible engines. The irreversibilities are assumed caused by heat transfer only. The specific power, defined as the power output per unit total heat exchanger surface area, is adopted as the objective function in determining power economics in this paper.     

Experimental investigation on the heat transfer characteristics of axial rotating heat pipes

The heat transfer performance of axial rotating heat pipes was measured under steady state at rotational speeds up to 4000 RPM, or a maximum centrifugal acceleration of 170g, and heat transfer rates up to 0.7 kW. A cylindrical and an internally tapered heat pipe with water as the working fluid were tested with different fluid loadings that ranged from 5% to 30% of the total interior volume. The measurements were used to characterize the effects of rotational speed, working fluid loading, and heat pipe geometry on the heat transfer performance. The internal taper on the condenser was found to significantly increase the heat transfer rate compared to the cylindrical case. A comparison between the test results and predictions from previous models showed that natural convection in the liquid film at the heat pipe evaporator plays an important role in the heat transfer mechanism at high rotational speeds.     

Modeling of thermal radiation in a participating media with conjugate heat transfer

Modeling a combination of thermal radiation and conjugate heat transfer in a three-dimensional rectangular domain which has a participating media CO₂ flowing through is done numerically in OpenFOAM. The rectangular duct has a vertical step (facing forward to the inlet) which is located at a distance from the inlet (the distance is same as the height of the inlet section). The domain is divided into two regions (namely solid and fluid). Carbon dioxide, a highly absorbing fluid with extinction, is used here as the participating medium. The ability of the code is verified to analyze the thermal radiation in a participating media with conjugate heat transfer. The study was carried out for a constant Reynolds number 250 and a contraction ratio of 0.5. The study focused primarily on the importance of adding thermal radiation on to thermal analysis and the reason behind the Nusselt number variation on different regions of solid–fluid interface. It also discussed the effect of radiative properties, such as optical thickness and linear scattering albedo, on the average convective Nusselt Number.     

Optimization of heat transfer device and analysis of heat & mass transfer on the finned multi-tubular metal hydride tank

Based on the previous studies on heat and mass transfer characteristics of hydride tank, whether the reaction heat of hydride bed can be removed quickly is a determinant factor of the reaction rate. As the core part of reaction system, the heat transfer optimization in the tank can significantly enhance the reaction rate. In this paper, the optimization of heat transfer fins for a finned multi-tubular metal hydride tank is presented, and the heat transfer equations of tank with various configuration fins (radius, thickness and number) are derived. By analyzing the effects of fin configurations on the heat transfer device, we found that the thermal resistance of reaction system reduces with the increase of the fin radius, thickness and number. In order to study transient reaction process inside the hydride tank with various configuration and operation conditions, a 3-D mathematical model is developed and validated based on the experimental data from literature. Through simulation and optimization on hydride tank with different configurations, we got that the fin number has the most significant positive effect on the absorption reaction process. The numerical simulation results show that the hydrogen absorption rate is proportional to hydrogen pressure, heat transfer coefficient and fluid flow velocity, and the hydrogen pressure has the most remarkable impact among these factors. The H₂ absorption is accomplished in 1720 s at 1 MPa, and the absorption reaction is completed within 2000 s at the H₂ pressure of 0.8 MPa. Moreover, the maximum difference in absorption completion time is only 190 s under different heat transfer coefficients and fluid flow velocities.     

节流型微通道换热特性研究

为研究节流型微通道换热特性,设计并加工制作了突缩突扩结构的微通道实验件。采用控制变量法控制改变加热电压、质量流量、入口温度,通过实验数据对比分析研究了影响节流型微通道对流换热的规律。研究结果表明:随着质量流量的增加,微通道蒸发器的对流传热系数不断减小;随着雷诺数的增大努谢尔数不断增大,对流换热效果比较明显。