Wall Thermal Boundary Conditions Research Articles

Shell-side thermal-hydraulic performances of multilayer spiral-wound heat exchangers under different wall thermal boundary conditions

The multilayer spiral-wound heat exchanger (SWHX) is used extensively in many industrial fields, but little research on the shell-side thermal-hydraulic performances in SWHXs has been performed. So an investigation on the shell-side flow and heat transfer performances of multilayer SWHXs under turbulent flow is implemented by using experimental and numerical methods. An experiment on the shell-side flow and heat transfer performance of a self-manufactured SWHX with three layers of coils is carried out under heat flux specified boundary condition (BC) to validate a numerical method. The results obtained by the simulations agree well with those from the experiment. Furthermore, to study the effects of different thermal BCs of the tube wall on the shell-side heat transfer performance in the multilayer SWHXs, numerical simulations are performed under the thermal BC of constant heat flux and constant temperature. Then the results are compared to those of the water-to-water conjugate heat transfer. It is found that the maximum relative deviation is 11.4% and 3.5%, respectively. Finally, the correlation of the shell-side Nusselt number, Nus is obtained by the Wilson plot method, which is Nus = 0.179·Res0.862, with the available range of Res from 500 to 3500.

Constrained Large-Eddy Simulation of Compressible Flow Past a Circular Cylinder

AbstractCompressible flow past a circular cylinder at an inflow Reynolds number of 2x105is numerically investigated by using a constrained large-eddy simulation (CLES) technique. Numerical simulation with adiabatic wall boundary condition and at a free-stream Mach number of 0.75 is conducted to validate and verify the performance of the present CLES method in predicting separated flows. Some typical and characteristic physical quantities, such as the drag coefficient, the root-mean-square lift fluctuations, the Strouhal number, the pressure and skin friction distributions around the cylinder,etc.are calculated and compared with previously reported experimental data, finer-grid large-eddy simulation (LES) data and those obtained in the present LES and detached-eddy simulation (DES) on coarse grids. It turns out that CLES is superior to DES in predicting such separated flow and that CLES can mimic the intricate shock wave dynamics quite well. Then, the effects of Mach number on the flow patterns and parameters such as the pressure, skin friction and drag coefficients, and the cylinder surface temperature are studied, with Mach number varying from 0.1 to 0.95. Non-monotonic behaviors of the pressure and skin friction distributions are observed with increasing Mach number and the minimum mean separation angle occurs at a subcritical Mach number of between 0.3 and 0.5. Additionally, the wall temperature effects on the thermodynamic and aerodynamic quantities are explored in a series of simulations using isothermal wall boundary conditions at three different wall temperatures. It is found that the flow separates earlier from the cylinder surface with a longer recirculation length in the wake and a higher pressure coefficient at the rear stagnation point for higher wall temperature. Moreover, the influences of different thermal wall boundary conditions on the flow field are gradually magnified from the front stagnation point to the rear stagnation point. Moreover, the influences of different thermal wall boundary conditions on the flow field are graduallymagnified from the front stagnation point to the rear stagnation point. It is inferred that the CLES approach in its current version is a useful and effective tool for simulating wall-bounded compressible turbulent flows with massive separations.

Impact of Wall Temperature on Turbine Blade Tip Aerothermal Performance

Currently the aerodynamics and heat transfer over a turbine blade tip tend to be analyzed separately with the assumption that the wall thermal boundary conditions do not affect the over-tip-leakage (OTL) flow field. There are some existing correlations for correcting the wall temperature effect on heat transfer when scaled to engine realistic conditions. But they were either developed to account for the temperature dependence of fluid properties largely empirically, or based on a boundary-layer model. It would be difficult (if not impossible) to define a boundary layer in many parts of a realistic blade passage with marked three-dimensional (3D) end wall and secondary flows (including those within a blade tip and around it). The questions to be asked here are: is the OTL aerodynamics significantly affected by the wall thermal condition? And if it is, how can we count this effect consistently in turbine blade tip design and analysis using modern CFD methods? In the present study the problem has been examined for typical high-pressure turbine blade tip configurations. An extensively developed RANS code (HYDRA) is employed and validated against the experimental data from a high speed linear cascade testing rig. The numerical analysis reveals that the wall–gas temperature ratio could greatly affect the transonic OTL flow field and there is a strong two-way coupling between aerodynamics and heat transfer. The feedbacks of the thermal boundary condition to aerodynamics behave differently at different flow regimes over the tip, clearly indicating a highly localized dependence of the convective heat transfer coefficient (HTC) upon wall temperatures. This implies that to use HTC for blade metal temperature predictions without resorting a fully conjugate solution, the temperature dependence needs to be corrected locally. A nonlinear correction approach has been adopted in the present work, and the results demonstrate its effectiveness for the transonic turbine tip configurations studied.

CFD modeling of buoyancy driven cavities with internal heat source—Application to heated rooms

The aim of this work is to examine the capacity and the accuracy of a CFD (Computational Fluid Dynamics) model to characterize the thermo-aeraulic behavior of a heated room. Firstly, we present a brief description of the experimental set-up taken into account (two experimental tests with changed room boundary conditions were taken into account). Afterwards, the focus is on the main features of the numerical model (that strongly influence the accuracy of results): computational domain geometry and discretization, turbulence model, near wall treatment, radiation model and thermal boundary conditions. In addition, a simplified approach is presented here in order to integrate a pure buoyancy source within the model, based on a volumetric heat generation rate which is uniformly distributed within the heater. Furthermore, detailed experimental–numerical comparisons are given with regard to heat transfer to the walls as well as to heat source behavior and plume characteristics. The results obtained demonstrate that the CFD method employed in this work leads to reliable results. Consequently, this approach can be useful in detailed studies dealing with thermal comfort, indoor air quality and energy consumption for heated rooms. Finally, the simplified method presented here, concerning the integration of the heat source in the CFD model, can be effortlessly extended for other localized heat sources that differ in power, heat emission mode (convection or radiation), shape or size.

Effects of wall conduction and rarefaction on shock propagation in a micro-channel

A Direct Simulation Monte Carlo code is coupled with a continuum unsteady conduction finite volume code to study the effects of wall conduction on shock propagation in a micro-channel. In addition, the effects of rarefaction and wall accommodation are also investigated. Wall accommodation is seen to influence the shock attenuation more significantly than wall conduction. The results indicate that both the shock front Mach number and shock strength are comparatively higher in case of adiabatic wall condition as there is no loss of energy from the shocked region. At higher rarefaction, the effect of wall accommodation as well as that of wall conduction are seen to become more noticeable. The simulation results show that it is not necessary to exactly model the wall conduction heat transfer for studying shock attenuation, and the two limiting situations of isothermal and adiabatic approximations are satisfactory as the thermal wall boundary conditions.

On the optimum mass transfer of flat absorbing falling films

The coupled heat- and mass-transfer of the absorbing falling film is considered in the flat-film approximation. We study the influence of the operating conditions on the absorption efficiency in a fully non-dimensional framework and find an optimal Reynolds number, which can be interpreted as an optimal film thickness. We also test various thermal wall boundary conditions to select the best approximation compared to a realistic test case.

Effects of thermal boundary conditions on the hypersonic shock layer flow over a blunt body

This paper presents a numerical study of the effects of thermal boundary conditions on the shock layer flow over a spacecraft entering a planetary atmosphere at hyper-velocity, and on the resulting convective heat fluxes to its heat shield surface. This study is motivated by the fact that spacecraft heat shields are covered with materials that ablate to reduce the spacecraft wall temperature. The ablation is not taken into account, but the effects of changes in the temperature of the vehicle surface are thoroughly studied. The results show that the flow field solutions within the shock layer and the convective heat fluxes are sensitive to the wall thermal boundary conditions. A computation of the flow field solution for an adiabatic wall is also performed. In this case, the gas recovery temperature along the wall increases as the gas moves away from the stagnation point. Computation convergence is also analysed.

Finite element based heatline approach to study mixed convection in a porous square cavity with various wall thermal boundary conditions

A penalty finite element method based simulation is performed to analyze the influence of various walls thermal boundary conditions on mixed convection lid driven flows in a square cavity filled with porous medium. The relevant parameters in the present study are Darcy number ( Da = 10 −5 − 10 −3), Grashof number ( Gr = 10 3 − 10 5), Prandtl number ( Pr = 0.7–7.2), and Reynolds number ( Re = 1–10 2). Heatline approach of visualizing heat flow is implemented to gain a complete understanding of complex heat flow patterns. Patterns of heatlines and streamlines are qualitatively similar near the core for convection dominant flow for Da = 10 −3. Symmetric distribution in heatlines, similar to streamlines is observed irrespective of Da at higher Gr in natural convection dominant regime corresponding to smaller values of Re. A single circulation cell in heatlines, similar to streamlines is observed at Da = 10 −3 for forced convection dominance and heatlines are found to emanate from a large portion on the bottom wall illustrating enhanced heat flow for Re = 100. Multiple circulation cells in heatlines are observed at higher Da and Gr for Pr = 0.7 and 7.2. The heat transfer rates along the walls are illustrated by the local Nusselt number distribution based on gradients of heatfunctions. Wavy distribution in heat transfer rates is observed with Da ⩾ 10 −4 for non-uniformly heated walls primarily in natural convection dominant regime. In general, exponential variation of average Nusselt numbers with Grashof number is found except the cases where the side walls are linearly heated. Overall, heatlines are found to be a powerful tool to analyze heat transport within the cavity and also a suitable guideline on explaining the Nusselt number variations.

CHARACTERISTICS OF THE FLOW AND HEAT TRANSFER OF GALLIUM IN A MINI-SCALE CHANNEL UNDER THE EFFECT OF MAGNETIC FIELD

The characteristics of the heat transfer processes in a mini-scale channel of gallium flow under the influence of a transverse uniform magnetic field with magnitude of 0.1 Tesla are investigated numerically. The effect of the wall thermal boundary condition and the location of its application on the boundary is investigated by adopting three such boundary condition locations applied separately and in combination. Two values of Reynolds number are considered. Results show a clear impact of the induced electromagnetic force on the flow and heat transfer between gallium and the channel walls. The effect of the magnetic field on the flow and heat transfer increase with increasing the Reynolds number. The wall thermal boundary condition and the location of its application on the channel boundary have great impact on the results.

Direct numerical simulation of transitional flow at high Mach number coupled with a thermal wall model

In transitional and turbulent high speed boundary-layer flows the wall thermal boundary conditions play an important role and in many cases an assumption of a constant temperature or a specified heat flux may not be appropriate for numerical simulations. In this paper we extend a formulation for direct numerical simulation of compressible flows to include a thin plate that is thermally fully coupled to the flow. Even without such thermal coupling compressible flows with shock waves and turbulence represent a challenge for numerical methods. In this paper we review the scaling properties of algorithms, based on explicit high-order finite differencing combined with shock capturing, that are suitable for dealing with such flows. An application is then considered in which an isolated roughness element is of sufficient height to trigger transition in the presence of acoustic forcing. With the thermal wall model included it is observed that the plate heats up sufficiently during the simulation for the transition process to be halted and the flow consequently re-laminarises.

A Novel Technique for Assessing Turbine Cooling System Performance

A new experimental technique for the accurate measurement of steady-state metal temperature surface distributions of modern heavily film-cooled turbine vanes has been developed and is described in this paper. The technique is analogous to the thermal paint test but has been designed for fundamental research. The experimental facility consists of an annular sector cascade of high pressure (HP) turbine vanes from a current production engine. Flow conditioning is achieved by using an annular sector of deswirl vanes downstream of the test section, being both connected by a three-dimensionally contoured duct. As a result, a transonic and periodic flow, highly representative of the engine aerodynamic field, is established: Inlet turbulence levels, mainstream Mach and Reynolds numbers, and coolant-to-mainstream total pressure ratio are matched. Since the fully three-dimensional nozzle guide vane (NGV) geometry is used, the correct radial pressure gradient and secondary flow development are simulated and the cooling flow redistribution is engine-realistic. To allow heat transfer measurements to be performed, a mainstream-to-coolant temperature difference (up to 33.5°C) is generated by using two steel-wire mesh heaters, operated in series. NGV surface metal temperatures are measured (between 20°C and 40°C) by wide-band thermochromic liquid crystals. These are calibrated in situ and on a per-pixel basis against vane surface thermocouples, in a heating process that spans the entire color play and during which the turbine vanes can be assumed to slowly follow a succession of isothermal states. Experimental surface distributions of overall cooling effectiveness are presented in this paper. By employing resin vanes of the same geometry and cooling configuration (to implement adiabatic wall thermal boundary conditions) and the transient liquid crystal technique, surface distributions of external heat transfer coefficient and film cooling effectiveness can be acquired. By combining these measurements with those from the metal vanes, the results can be scaled to engine conditions with a good level of accuracy.

Three-dimensional modelling of pneumatic drying process

Steady-state three-dimensional calculations of heat and mass transfer in vertical pneumatic dryer were performed. The theoretical model of the drying process is based on two-phase Eulerian–Lagrangian approach for gas-particles flow and incorporates advanced drying kinetics for wet particles. The model was utilized for simulation of the drying process of wet PVC and silica particles in a large-scale vertical pneumatic dryer. The influence of wall thermal boundary conditions was investigated by assuming either known value of the wall temperature or adiabatic flow in the dryer. Analyzing the predicted particle drying kinetics, an uneven product quality was predicted due to non-uniform drying conditions in the central and peripheral zones of the pneumatic dryer. Moreover, for the case of non-insulated chamber walls such quality unevenness was estimated to be substantially greater than for the case with thermally insulated drying chamber. The examination of the predicted temperature profiles within the silica and PVC wet particles showed that the latter is subjected to higher temperature gradients potentially resulting in the greater rate of thermally-degraded final product.

Development and Validation of a Thickened Flame Modeling Approach for Large Eddy Simulation of Premixed Combustion

The development of a dynamic thickened flame (TF) turbulence-chemistry interaction model is presented based on a novel approach to determine the subfilter flame wrinkling efficiency. The basic premise of the TF model is to artificially decrease the reaction rates and increase the species and thermal diffusivities by the same amount, which thickens the flame to a scale that can be resolved on the large eddy simulation (LES) grid while still recovering the laminar flame speed. The TF modeling approach adopted here uses local reaction rates and gradients of product species to thicken the flame to a scale large enough to be resolved by the LES grid. The thickening factor, which is a function of the local grid size and laminar flame thickness, is only applied in the flame region and is commonly referred to as dynamic thickening. Spatial filtering of the velocity field is used to determine the efficiency function by accounting for turbulent kinetic energy between the grid-scale and the thickened flame scale. The TF model was implemented into the commercial computational fluid dynamics code FLUENT. Validation in the approach is conducted by comparing model results to experimental data collected in a laboratory-scale burner. The burner is based on an enclosed scaled-down version of the low swirl injector developed at Lawrence Berkeley National Laboratory. A perfectly premixed lean methane-air flame was studied, as well as the cold-flow characteristics of the combustor. Planar laser induced fluorescence of the hydroxyl molecule was collected for the combusting condition, as well as the velocity field data using particle image velocimetry. Thermal imaging of the quartz liner surface temperature was also conducted to validate the thermal wall boundary conditions applied in the LES calculations.

Benchmark Studies for the Generalized Streamwise Periodic Heat Transfer Problem

This is a comparison of calculations performed with a scheme for handling streamwise-periodic boundary conditions with known solutions to the common problem of fully developed heat transfer in a plane duct. Constant value, constant flux, mixed boundary conditions, and linear wall flux (conjugate heat transfer) are all considered. Agreement is, in every case, near exact showing that the methodology may be applied with confidence to complex engineering problems with a variety of thermal wall boundary conditions.

Numerical Study of Pulsed Turbulent Plane Wall Jet by Low-Reynolds-Number k–ϵ Model

The present study reports on the numerical characterization of the influence of an initial perturbation in the development of turbulent plane two-dimensional wall jets. At the nozzle exit, the pulsation is imposed by a vertical component velocity: u = u 0[1 + Asin(ω t)]. In the configuration investigated, the jet may be either isothermal or submitted to various wall thermal boundary conditions: uniform temperature in mixed-convection regime or uniform heat flux in forced-convection regime. Numerical modeling of the pulsed wall jet is performed using a low-Reynolds number k–ϵ model. A finite-difference method, sing a staggered grid, is employed to solve the coupled governing equations associated to the inlet and boundary conditions. The effects of the parameters characterizing the pulsation, such as the amplitude and the frequency, on the dynamic, turbulent, and thermal characteristics of the turbulent wall jets are investigated in detail.

Effect of different thermal wall boundary conditions on compressible turbulent channel flow at M =1.5

The main objective of this study is to clarify the effect of thermal wall boundary conditions on turbulence statistics and structures in a compressible turbulent flow. This work is an extension of Morinishi et al. (J. Fluid Mech. vol. 502, 2004, p. 273), who performed DNS of compressible turbulent channel flow between adiabatic and isothermal walls at Mach number (Case 1) and DNS of the corresponding incompressible turbulent flow with passive scalar transport (Case I) are performed. The present study shows that the mean temperature profile near the high-temperature wall for Case 1 has an additional maximum due to the friction work, while such an additional maximum does not appear for Cases 2 and I. The additional maximum leads to a corresponding near-wall maximum of temperature fluctuations. We find the direction of energy transfer due to pressure work near the adiabatic wall for Case 2 being opposite to that near the isothermal wall to be due to the effect of the high-temperature wall, not to the effect of the adiabatic wall. These findings are explained by using the budgets of internal energy and temperature variance transport equations.

CHARAGERIZATION OF THE DISCHARGE COEFFICIENT OF A SONIC VENTURI NOZZLE

This paper identifles and determines the main parameters used to calculate the dlscharge coefficlent of a toroidal Venturl nozzle accordlng to the ISO Standard 9300, operating at the critical flow regime (sonlc). Thls study was conducted to investigate the effeds of the viscous stresses In the turbulent boundary layer, the wall thermal boundary condition, and the flow fleld curvature at the nucleus of the nozzle on the dlscharge coefflclent by means of a theoretical and numerlcal approach. Characterlzation of the dlscharge coefflclent in the Venturi sonlc nozzle was performed consldering the viscous and multidlmenslonal effects of the flujd flow as uncoupled phenomenon. As a result. each non-ideal mechanism can be analyzed independently from the Inftuence of the other mechanlsm. We present a numerlcal procedure to characterlze the dlscharge coefflclent In the invlscld reglon of the ftow, by uslng the numerical simulation of the inviscld maln ftow by means of the commerclal CFD code. In the region of the vlscous stresses, the charaderlzation of thls coefflclent is based on the analytlcal theory of the turbulent boundary layer. Thls charaderization allowed obtalning a correlation of the dlscharge coefflclent that was valldated by dlred comparlson between the experimental correlations of the dlscharge coefflclent In turbulent boundary layer proposed by ISO-9300 and the Korea Research Institute of Standards and Sclence (KRISS). Thls valldation was carried out for throat Reynolds numbers from 1.4 to 2.6x10'. The agreement of the theoretical and measured dlscharge coefflclents by these correlations was better than 0.2%.

Comparison of a CFD Fire Model against a Ventilated Fire Experiment in an Enclosure

Computer modelling of fire has become an attractive approach for the fire safety assessment of proposed building structures. To devise and validate the fire model, a general–purpose computational fluid dynamics (CFD) software package, CFX, has been evaluated against a fire test case in a ventilated room. The test room is 6.0 m long, 4.0 m wide and 4.5 m high with an exit opening of 0.65 m x 0.65 m. A simple inert fire model is used, in which a constant volumetric heat release is introduced at the location of the fire source to represent the fire. Combustion chemical reactions are not included in the computation; there was no flame spread and flashover to the wall lining material in this experiment, as the wall lining was non–combustible. The heat contribution is solely from the burner. It was demonstrated that both the k– ε model and the Shear Stress Transport (SST) hybrid turbulence model are capable of predicting the fire–generated turbulent flow and heat transfer. The computational result has an error of about 20 °C when compared with the measured gas temperature of the Lawrence Livermore National Laboratory (LLNL), in which the heat release from a gas burner is used to represent a fire. It has been confirmed that the thermal energy deposit into the wall plays a significant role in the whole transient process. From the numerical point of view, the exterior wall thermal boundary condition treatment has little influence on the fire inside the room, as at 20 minutes after the start of the fire, heat penetrates only about 3 – 4 centimetres into the 20–cm–thick wall in this experimental case. The energy budget has been analysed to understand the energy transfer for this fire test case. According to this test, about 30 percent of the heat from the fire is released by thermal radiation, and about 30 percent is carried out of the room by the ventilation air over the first 20 minutes after ignition, the rest is deposited into the walls, ceiling, and the floor.It can be concluded that this CFD approach can serve as a tool for the modelling of fire generated heat transfer in an enclosure. Thermal radiation plays an important role in the heat transfer process from the fire. It has been concluded that, in order to accurately simulate a fire case, the conjugate heat transfer must be included in the fire mathematical model.

Scalar and joint velocity–scalar PDF modelling of near-wall turbulent heat transfer

The temperature field in a heated turbulent flow is considered as a dynamically passive scalar. The probability density function (PDF) method with down to the wall integration is explored and new modelling proposals are put forward, including the explicit account for the molecular transport terms. Two variants of the approach are considered: first, the scalar PDF method with the use of externally-provided turbulence statistics; and second, the joint (stand-alone) velocity–scalar PDF method where a near-wall model for dynamical variables is coupled with a model for temperature. The closure proposals are formulated in the Lagrangian setting and resulting stochastic evolution equations are solved with a Monte Carlo method. The near-wall region of a heated channel flow is taken as a validation case; the second-order thermal statistics are of a particular interest. The PDF computation results agree reasonably with available DNS data. The sensitivity of results to the molecular Prandtl number and to the thermal wall boundary condition is accounted for.

Numerical Analysis of Compressible Turbulent Channel Flow with Wall-Temperature Difference

The objective of this study is to clarify the effect of different thermal wall boundary conditions on turbulence statistics and structures in the compressible turbulent flow. We performed direct numerical simulation (DNS) of the compressible turbulent channel flow between isothermal walls with temperature difference (Case C1). The DNS data was compared with that of the compressible turbulent channel flow between adiabatic and isothermal walls (Case C2), and that of the incompressible turbulent channel flow between isothermal walls with temperature difference (Case I). The main results are as follows : (1) The mean velocity in the logarithmic region and the maximum of the RMS streamwise velocity fluctuation in wall units are smaller than those of Case I near the high temperature isothermal wall, while they are larger than those of Case I near the adiabatic wall. (2) Near the high temperature isothermal wall, the mean temperature profile has an additional maximum due to the friction work, and the RMS temperature fluctuation has the corresponding maximum. This is consistent with the fact the production term of temperature variance has the additional maximum in the region very close to the high temperature isothermal wall. (3) The correlation coefficient between streamwise velocity and temperature fluctuations of Case C1 is significantly different from those of Cases C2 and I in the region very close to the high temperature wall. (4) The direction of energy transfer due to pressure work near the high temperature isothermal wall is the same as that of near the adiabatic wall. (5) The compressible terms of the temperature variance transport equation are not negligible near the low temperature isothermal wall.