Large eddy simulation of turbulent open channel flow with heat transfer at high Prandtl numbers

Summary. Large eddy simulation of a fully developed turbulent open channel flow with heat transfer is performed. The three-dimensional filtered Navier-Stokes and energy equations are numerically solved using a fractional-step method. Dynamic subgrid-scale (SGS) models for the turbulent SGS stress and heat flux are employed to close the governing equations. The objective of this study is to analyze the behavior of turbulent flow and heat transfer in turbulent open channel flow, in particular for high Prandtl number, and to examine the reliability of the LES technique for predicting turbulent heat transfer near the free surface. The turbulent open channel flow with constant difference of temperature imposed on the free surface and bottom wall is calculated for the Prandtl number (Pr) from 1 up to 100, the Reynolds number (Re) 180 based on the wall friction velocity and the channel depth. To illustrate the turbulent flow and heat transfer behaviors, some typical quantities, including the mean velocity, temperature and their fluctuations, heat transfer coefficients, turbulent heat fluxes, and flow structures of velocity and temperature fluctuations, are exhibited and analyzed.     

Temperature distribution in laminar flow of an incompressible fluid in a rectangular channel allowing for energy dissipation

Expressions have been obtained for the temperature distribution over the section and heat flux through the channel wall in the presence of energy dissipation for the case of laminar flow of a liquid in an Infinitely long channel of rectangular section with constant wall temperature.     

Two-Phase Flow and Heat Transfer During Chilldown of a Simulated Flexible Metal Hose Using Liquid Nitrogen

For many industrial, medical and space technologies, cryogenic fluids play irreplaceable roles. When any cryogenic system is initially started, it must go through a transient chill down period prior to normal operation. Chilldown is the process of introducing the cryogenic liquid into the system, and allowing the system components to cool down to several hundred degrees below the ambient temperature. The chilldown process is an important initial stage before a system begins functioning. The objective of this paper is to investigate the chilldown process associated with a flexible hose that was simulated by a channel with saw-teeth inner wall surface structure in the current study. We have investigated the fundamental physics of the two-phase flow and quenching heat transfer during cryogenic chilldown inside the simulated flexible hose through flow visualization, data measurement and analysis. The flow pattern developed inside the channel was recorded by a high speed camera for flow pattern investigation. The experimental results indicate that the chilldown process that is composed of unsteady vapor-liquid two-phase flow and phase-change heat transfer is modified by the inner wall surface wavy structure. Based on the measurement of the channel wall temperature, the teeth structure and the associated cavities generally reduce the heat transfer efficiency compared to the straight hose. Furthermore, based on the measured data, a complete series of correlations on the heat transfer coefficient for each heat transfer regime was developed and reported.     

Numerical Investigation of Unsteady Heat Exchange in Ignition for Reactive Channel Walls by the Flow of a High-Temperature Viscous Gas

A study is made of the process of ignition of reactive channel walls by a laminar flow of hot gases, including the stages of heating of a substance and of reacting in the surface layer with self-acceleration of the chemical reaction. The process is determined by the heat exchange between the gas and the wall, the strength of the heat source in the chemical-reaction zone, and the sink of heat due to conduction in the radial and axial directions. In the stage of self-heating, we can have heat sink not only deep into the wall and/or through its external boundary but into the gas flow as well. The problem has been solved in a conjugate formulation. The influence of the temperature, the velocity of the gas at the entrance to the channel, and the wall thickness on ignition characteristics has been studied.     

Direct numerical simulation of stably and unstably stratified turbulent open channel flows

Summary Direct numerical simulation of stably and unstably stratified turbulent open channel flow is performed. The three-dimensional Navier-Stokes and energy equations under the Boussinesq approximation are numerically solved using a fractional-step method based on high-order accurate spatial schemes. The objective of this study is to reveal the effects of thermally stable and unstable stratification on the characteristics of turbulent flow and heat transfer and on turbulence structures near the free surface of open channel flow. Here, fully developed weakly stratified turbulent open channel flows are calculated for the Richardson number ranging from 20 (stably stratified flow) to 0 (unstratified flow) and to −10 (unstably stratified flow), the Reynolds number 180 based on the wall friction velocity and the channel depth, and the Prandtl number 1. To elucidate the turbulent flow and heat transfer behaviors, typical quantities including the mean velocity, temperature and their fluctuations, turbulent heat fluxes, and the structures of velocity and temperature fluctuations are analyzed.     

Effects of nanoparticle migration on force convection of alumina/water nanofluid in a cooled parallel-plate channel

Force convective heat transfer of alumina/water nanofluid inside a cooled parallel-plate channel in the creeping flow regime and the presence of heat generation is investigated theoretically. A modified two-component four-equation non-homogeneous equilibrium model is employed for the alumina/water nanofluid that fully accounts for the effects of nanoparticles volume fraction distribution. To impose the temperature gradients across the channel, the upper wall is subjected to a prescribed wall heat flux while the bottom wall is kept adiabatic. Moreover, due to the nanoparticle migration in the fluid, the no-slip condition of the fluid–solid interface at the walls is abandoned in favor of a slip condition that appropriately represents the non-equilibrium region near the interface. The results indicated that nanoparticles move from the adiabatic wall (nanoparticles depletion) toward the cold wall (nanoparticles accumulation) and construct a non-uniform nanoparticle distribution. Moreover, the anomalous heat transfer rate occurs when the Brownian motion takes control of the nanoparticle migration (smaller nanoparticles).     

Conjugate heat transfer in solar collector panels with internal longitudinal corrugated fins-Part II: Local results

A computational study is conducted to determine, for laminar flow in a solar collector panel or a parallel-plate channel with an internal, longitudinal, corrugated fin, the effects of varying the fin angle (or the fin pitch), the fin thickness, the wall-to-fluid thermal conductivity ratio, and the thermal boundary condition on the local surface temperature and heat flux distributions. The corrugated fin and the two walls form individual triangular flow passages in the collector panel or parallel-plate channel. The results of the investigation show that the variation of the local surface temperature is large when the fin is thin and when the wall/fluid thermal conductivity ratio is small. The local surface heat flux is low near the corners of both the upper and lower triangular flow passages. Near the point of fin attachment on the heated wall, heat may be transferred from the fluid to the fin. Heat may also be transferred from the fluid to the unheated wall near the point of fin attachment. When the thermal conductivity ratio is small, the temperature field in the flow cross section is predominantly stratified. In the limit as the thermal conductivity ratio approaches infinity, the temperature field is that of the thermally fully developed laminar flow in a triangular duct with a streamwise uniform heat flux and peripheral uniform surface temperature boundary condition.     

Heat transfer in the thermally stabilized sections of channels with a laminar non-newtonian fluid flow

An approximate method is described for solving the problem of heat transfer for laminar non-newtonian fluid flow in the thermally stabilized sections of a plane channel and a circular tube under conditions of constant wall temperature.     

The effect of wall conductance on heat diffusion in duct flow

Summary The axial diffusion of a passive scalar field (e.g. temperature) in Poiseuille flow through a duct is considered, taking account of leakage of heat through the duct boundary. The cases of the two-dimensional channel and the pipe of circular cross-section are considered in detail, and it is shown that (i) the centroid of the scalar field moves (asymptotically) with a velocity intermediate between the mean and the maximum flow rates and increases with increasing wall conductance, and (ii) the effective diffusivity in the flow direction is a decreasing function of wall conductance.The temperature field downstream of a maintained heat source is determined as a function of wall conductance.     

Two-Phase Thermal Transport in Microgap Channels—Theory,Experimental Results,and Predictive Relations

A comprehensive literature review and analysis of recent microchannel/microgap heat transfer data for two-phase flow of refrigerants and dielectric liquids is presented. The flow regime progression in such a microgap channel is shown to be predicted by the traditional flow regime maps. Moreover, Annular flow is shown to be the dominant regime for this thermal transport configuration and to grow in importance as the channel diameter decreases. The results of heat transfer studies of single miniature channels, as well as the analysis and inverse calculation of IR images of a heated microgap channel wall, are used to identify the existence of a characteristic M-shaped heat transfer coefficient variation with quality (or superficial velocity), with inflection points corresponding to transitions in the two-phase cooling modalities. For the high-quality, Annular flow conditions, the venerable Chen correlation is shown to yield predictive agreement for microgap channels that is comparable to that attained for macrochannels and to provide a mechanistic context for the thermal transport rates attained in microgap channels. Results obtained from infrared imaging, revealing previously undetected, large surface temperature variations in Annular flow, are also reviewed and related to the termination of the favorable thin-film evaporation mode in such channels.     

Simulations of flow through fluid/porous layers by a characteristic‐based method on unstructured grids

An upwind characteristic‐based finite volume method on unstructured grids is employed for numerical simulation of incompressible laminar flow and forced convection heat transfer in 2D channels containing simultaneously fluid layers and fluid‐saturated porous layers. Hydrodynamic and heat transfer results are reported for two configurations: the first one is a backward‐facing step channel with a porous block inserted behind the step, and the second one is a partially porous channel with discrete heat sources on the bottom wall. The effects of Darcy numbers on heat transfer augmentation and pressure loss were investigated for low Reynolds laminar flows. The results demonstrate the accuracy and robustness of the numerical scheme proposed, and suggest that partially porous insertion in a channel can significantly improve heat transfer performance with affordable pressure loss. Copyright © 2001 John Wiley & Sons, Ltd.     

Experiments on dry-out during flow boiling in a round minichannel

Temperature measurements during flow boiling of R134a in a 0.96 mm single circular channel are reported in order to provide a criterion for the determination of the critical conditions in the channel. The flow boiling heat transfer is obtained by using a secondary fluid; the wall temperature displays larger fluctuations in the zone where dryout occurs. These temperature fluctuations in the wall denote the presence of a liquid film drying up at the wall with some kind of an oscillating process. These temperature fluctuations never appear during condensation tests, neither are present during flow boiling at low vapour qualities. The fluctuations also disappear in the post-critical condition zone. Experimental values of dryout quality measured with the above method are reported in this paper at mass velocity ranging between 300 and 600 kg m^?2s^?1. In the practical applications of flow boiling, the dryout quality is a key parameter in the two-phase systems for cooling of devices, both for ground and microgravity applications. The test conditions reported here refer to relatively high mass velocities, and are obtained at earth gravity. Nevertheless, since the critical heat flux differences between the two gravity environments decrease with increasing velocity, the present data may also be used for inertia dominated systems at low g.     

An experimental investigation of heat transfer in a pebble bed

An experimental investigation is performed of heat transfer under conditions of longitudinal flow of water moving in a bed of glass pebbles past a flat heated wall. Experiments involving the flow of single-phase liquid are performed in the ranges of variation of filtration velocities from 6 to 60 mm/s and of heat fluxes from 40 to 140 kW/m²; in the case of wall boiling, the experiments are performed in the ranges of variation of filtration velocities from 2 to 50 mm/s and of heat fluxes from 27 to 86 kW/m². The temperature distribution is measured over the height of the heated wall and over the cross section (depth) of the channel at the outlet from the pebble bed. The experimental data are processed for single-phase flow using numerical optimization techniques. The values of the coefficient of “turbulent” thermal conductivity in the pebble bed are obtained as a function of the process parameters     

Self-oscillations of nonisothermal flow of viscous liquid in a channel

The process of emergence of self-oscillations of a flow of viscous liquid in a channel under conditions of heat transfer to the cold channel wall is considered. The parameters of liquid and heat transfer, at which self-oscillations may exist, are obtained, as well as the dependences of the period and amplitude of self-oscillations on the parameters of the system. The behavior of the system is investigated in the vicinity of the critical point determined in the curve of the dependence of liquid flow rate on pressure difference. A generalizing scheme of operation is given for a number of self-oscillatory systems, and it is demonstrated that their behavior in the vicinity of the critical point is described by the same equations.     

Laminar Boundary Layer in the Channels of Nuclear-Pumped Flowing Lasers

The paper deals with the analytical solution of the problem on the laminar hydrodynamic thermal boundary layer with internal heat release, formed in a flat channel with a nonuniform wall temperature under conditions of steady-state flow of compressible gas irradiated by fission fragments. Subsonic velocities of pumping flow are treated, which are much lower than the velocity of sound.     

Heat and mass transfer characteristics of a constrained thin-film ammonia-water bubble absorber

A study of absorption of ammonia vapour bubbles into a constrained thin-film of ammonia-water solution is presented. A large-aspect-ratio microchannel constrains the thickness of the weak solution film and ammonia vapour bubbles are injected from a porous wall. A counter flowing coolant in a minichannel removes the generated heat of absorption. Experiments and a simple one-dimensional numerical model are used to characterize the absorber performance at a nominal system pressure of 6.2 bar absolute. Effect of varying the mass flow rate of the weak solution, vapour flow rate, solution inlet temperature, and coolant inlet temperature on absorption heat and mass transfer rates and exit subcooling are discussed. Two absorber channel geometries, each of 600 μm nominal depth, are considered: 1) a smooth-wall channel, and 2) a stepped-wall channel that has 2-mm deep trenches across the width of a channel wall. Results indicate that the reduction in coolant inlet temperature significantly enhances the mass transfer rates in both absorber geometries. While the stepped-wall geometry exhibits higher mass transfer rates at lower coolant inlet temperatures of 30 °C and 40 °C, the smooth-wall channel shows higher mass transfer rates at the highest coolant inlet temperature of 58 °C. Both absorption limited and residence time limited conditions are observed with variation of weak solution flow rate at fixed vapour flow rates.     

Hydromagnetic flow and heat transfer of a second-grade viscoelastic fluid in a channel with oscillatory stretching walls: application to the dynamics of blood flow

The characteristics of flow and heat transfer of a fluid in a channel with oscillatory stretching walls in the presence of an externally applied magnetic field are investigated. The fluid considered is a second-grade viscoelastic electrically conducting fluid. The partial differential equations that govern the flow are solved by developing a suitable numerical technique. The computational results for the velocity, temperature and the wall shear stress are presented graphically. The study reveals that flow reversal takes place near the central line of the channel. This flow reversal can be reduced to a considerable extent by applying a strong external magnetic field. The results are found to be in good agreement with those of earlier investigations.     

Numerical study of flow and heat transfer of superfluid helium in capillary channels

A modified two-fluid model is adopted to study flow and heat transfer of superfluid helium in a microchannel with a diameter as small as that of a superleak in a fountain effect pump. Variable properties of superfluid helium and energy dissipations due to the two-fluid mutual friction and the friction at the channel wall are fully taken into consideration. It is found that the normal fluid component flow is not trivial even in a channel with diameter of a micrometre, and that there exists an optimum diameter for the maximum mass flow rate. The flow of superfluid helium through a channel with different temperatures at the ends differs considerably from that of a Newtonian fluid. The strong dependence of the thermodynamic properties on temperature and pressure, as well as the internal-convection mechanism are found to be the causes of the unique flows.     

冷凝器冷却水通道声传递特性

为阐明冷凝器冷却水通道的声传递特性、提高循环水系统声学设计能力,将换热方程和一维平面波方程耦合,推导得到换热管内冷却水声传递矩阵,针对冷凝器几何结构建立总传递矩阵并求解得到其冷却水通道声传递损失。建立试验系统验证了冷凝器冷却水管路声传递损失计算结果。根据换热管双向流固耦合分析计算结果,管外蒸汽绕流对换热管内冷却水脉动压力的影响可以忽略,冷凝器进出口管内水声和管壁振动测试结果也表明,该系统内冷却水脉动和管壁振动耦合紧密,管内流体脉动是管壁振动的主要激励源。研究结果还表明,通过调整冷凝器冷却水通道结构参数可以调节冷却水声传递损失。     

Flow of a slightly rarefied gas in a narrow slit channel in the case of sublimation from one of the walls and heat inflow from the other

The flow of a slightly rarefied gas (Kn<0.1) in a narrow slit channel during sublimation from one of the walls under the influence of an inflow of heat (by conduction and radiation) from the other wall of the channel is considered. An equation is derived for the pressure distribution in narrow slit channels in a form convenient for analytical investigation.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 28, No. 1, pp. 46–56, January, 1975.