Region Heat Transfer Research Articles

Thermal radiative study of counterflow combustion of porous particles

The paper deals with modeling counterflow, non-premixed combustion of porous fuel particles (lycopodium) with the consideration of thermal radiation effects. Assuming that the streams of fuel particles and air as oxidizer, move towards the stagnation plane from the two opposing nozzles in a counterflow configuration. It is presumed that particles first vaporize in order to yield a gaseous fuel, methane, which then reacts with the oxidizer which is air. In this research, conservation equations with certain boundary conditions are solved using mathematical methods with the consideration of radiation heat transfer in different regions and compared to cases in which radiation heat transfer is not considered. Furthermore, flame temperature and mass fraction profiles are presented in terms of oxidizer and fuel Lewis numbers. In addition, effects of particle porosity are investigated. As a result, with the increase of dust concentration and reduction of particles radius and porosity, it would lead to a rise in the flame temperature.

Study on the performance of indirect air-cooled system influenced by environmental temperature

In order to explore the mechanism of the performance of Air-cooled system influenced by environmental temperature, indirect air-cooled towers with surface condensers in power plant were studied in this paper. Through a proper simplification, the geometric models of indirect air-cooled towers and other major buildings around which may affect the fluid flow were established. When it comes to the air-cooled towers, which were located in the central region of flow and heat transfer, the structure type of the towers and the radiators should be clearly presented. Flow resistance and heat transfer characteristics of radiators were correctly and objectively taken into account, the reliability of which was verified by field test. Numerical simulation study on the thermal performance of the indirect air-cooled system with surface condensers was conducted by creating corresponding mathematical and physical models on the condition of designed wind director and velocity. At the same time, the thermal performance of indirect air-cooled towers under different environmental temperature was studied. The results showed that, the volume flow rate of air into the indirect cooling tower under different environmental temperature is basically unchanged. With temperature rising up, the density of air reduces, which leaded to a decrease on the mass flow rate of air into tower. Besides, the trends and changing laws of performance parameters from the level of segments and the tower are achieved. What’s more, the flow traces of air and flue gas in towers were basically similar which were clearly divided by a temperature line, but the two flows always melt into a whole one at the export of towers. The conclusions would be of guide significance to the optimal operation and design of indirect air-cooled system.

OpenFOAM based LES of slot jet impingement heat transfer at low nozzle to plate spacing using four SGS models

The objective of the present investigation is to assess the effectiveness of large eddy simulation (LES) in turbulent slot jet impingement heat transfer at low nozzle to plate spacing. Four different sub-grid stress (SGS) models, namely, Smagorinsky, WALE (wall adapting local eddy-viscosity), k-equation and dynamic k-equation, were considered for Reynolds number of 20,000. Computations were performed using OpenFOAM, an open source finite-volume based CFD code. Time and span-wise averaged mean streamwise velocity and root mean square (r.m.s.) velocity fluctuations in the stagnation and wall jet regions are presented. Nusselt number distributions on the impingement wall are also presented. The computed LES results are compared with the reported experimental data. A secondary peak in the Nusselt number was observed using the four SGS models as in the experimental data. LES of slot jet impingement heat transfer using four SGS models, including WALE, has been investigated for the first time in the present paper. It is observed that the WALE and dynamic k-equation SGS models perform well in complex flow regions of turbulent slot jet impingement heat transfer.

Performances and modelling of a circular moving bed thermochemical reactor for seasonal storage

A novel thermochemical reactor was designed, built, and tested at Besol’s and CEA-INES’ labs. Its circular shape and its vibrating bed allow to move the solid hydrate constantly, therefore increasing turbulence in the moist air heat and mass transfer region. A reactor model was developed and identified in order to calculate its performances over a wide range of operating conditions and to understand what are the key factors leading to increased performances, especially regarding the specific behavior of the composite material made of calcium chloride incorporated in a silica gel matrix. New kinetic equations were developed while combining sorption and porous medium physical phenomena. The model was further refined with a 10 layers solid bed spatial discretization. However vibrations actually mix those layers, which would require a more detailed approach. Nevertheless, outlet parameters were predicted in both modes with a deviation lower than 0.72 K equivalent.Test results in realistic conditions showed an average 356 W heating power with an air temperature elevation of +6.0 K, while desorption cooling power was 278 W with an average 4.5 K temperature decrease. The usable energy density with this 0.163 m3 uninsulated reactor was 200.4 W h per kg of 9% hydrated solid composite, which is adapted to seasonal storage since it is close to the 207.8 W h per kg theoretical target. Electrical consumption for air circulation is only about 10 W, but vibration accounts for almost 70 W, which still needs to be reduced. Detailed results showed that a continuous solid flow and a counter current configuration would lead to further increase average outlet temperature.

Effects of a pocket cavity on heat transfer and flow characteristics of the endwall with a bluff body in a gas turbine engine

A pocket cavity is generated at the connection of two parts, namely the transition part between the Low Pressure Turbine (LPT) and Outlet Guide Vane (OGV) in a gas turbine engine. This kind of pocket cavities, due to the high Reynolds number and the specific shapes, are hardly investigated in previous researches. In the present work, the effect of the pocket on the heat transfer of the endwall with a bluff body in the rear part of a gas turbine engine is investigated. A triangular pocket cavity is built in a rectangular channel and two kinds of bluff bodies, a cylinder or a cuboid is respectively put on the endwall. Liquid Crystal Thermography (LCT) is employed to measure the heat transfer over the endwall at Reynolds number ranging from 87,600 to 219,000. The turbulent flow details are presented by numerical calculations with the turbulence model, Detached Eddy Simulation (DES). For the pocket channel with a bluff body, the large heat transfer areas are usually found at the downstream edge of the pocket cavity and the vortices shedding regions around the bluff body. For the cuboid case, the high heat transfer regions are not at the leading edge of the cuboid, but are distributed as two banded regions upstream of the cuboid off the centerline. When a pocket cavity is placed in the upstream of the bluff bodies, cylinder or cuboid, the high heat transfer areas around the bluff bodies are decreased. The angle of the main flow attachment is changed due to the disturbances of the cavity. Accordingly, the flow impingement on the bottom wall of the bluff body is weakened and the heat transfer around the bluff body is decreased. The research displays the influence of incoming flows on the bluff body in the downstream and provides some references for placing OGV in the rear part of a gas turbine engine.

Effects of the pocket cavity on heat transfer and fluid flow of the downstream outlet guide vane at different flow attacking angles

A pocket cavity is generated at the junction position of the low pressure turbine (LPT) and the outlet guide vane (OGV) in the rear part of a modern gas turbine jet engine. In the present study, a triangular pocket cavity is placed upstream of an OGV at different distances. The effects of the pocket cavity on heat transfer and fluid flow of the downstream OGV with different flow attack angles are investigated numerically with well validated turbulence models. The flow attack angles are varied as –30°, 0°, and +30° at a constant Reynolds number =160,000. The turbulent flow details are provided by numerical calculations using two turbulence models, the unsteady DES model and the steady k-ω SST model. For different flow attack angles, the high Nusselt number regions around the OGV are changed. The high heat transfer region is really drawn back at a flow attack angle = +30° (Case 2b) compared with Case 2a with a flow attack angle =0°. As the flow attack angle is changed to –30° (Case 2c), the high Nusselt number regions are greatly enlarged not only on the suction side also on the pressure side because of the strengthened flow impingement on the vane surfaces. The pocket cavity weakens the flow impingement on the vane surfaces and the effect is more obvious when the pocket cavity is placed close to the vane. In addition, the heat transfer distribution over the pocket surface is also affected by the location of the vane. When the vane is placed close to the pocket cavity (Case 1), the heat transfer on the pocket edge is increased. In the case with a flow attack angle =0°, the high turbulent kinetic energy region is mainly located near the vane and wake region downstream the vane and recirculating flows can hardly be found.

Multiple-jet impingement heat transfer in double-wall cooling structures with pin fins and effusion holes

In a double-wall cooling system with multiple jet impingement the arrangement and size of effusion holes may change flow field and thereby change heat transfer characteristics. This paper stands in the view of internal cooling of turbine blades, and studied multiple-jet impingement heat transfer in double-wall cooling structures with narrow channels with pin fins and different-size effusion holes on the target wall. Five target plates were investigated including flat plate, pin fin plate and three pin fin plates with different-size effusion holes (effusion-to-jet diameter ratio of De/Dj=0.5,1.0and1.5). Transient liquid crystal thermography experiments were conducted to explore the heat transfer characteristics on these target plates. The ratio of jet-to-plate spacing was fixed to be 1.5 and Reynolds numbers based on the jet diameter range from 15,000 to 30,000. The experimental results showed that the pin fins and effusion holes reduce the crossflow strength in downstream region, improve and uniform heat transfer on the whole target plate obviously. Compared with the flat plate, pin fin plate with effusion holes of De/Dj=1.5 has highest averaged Nusselt number on the endwall. Numerical computations were carried out based on the experimental model, which revealed that the total heat transfer quantity on the pin fin plate with effusion holes of De/Dj=1.5 can be increased by up to 51% comparing to that of the flat plate. Detailed interactional flow information between the wall jet flow, pin fins and effusion holes is expounded for the heat transfer improvement in the impingement-effusion structures. Moreover, conjugate heat transfer analyses were done to further investigate the overall cooling performance of the impingement-effusion structures.

Discrete vs. continuum-scale simulation of coupled radiation and convection inside rectangular channel filled with metal foam

Open-cell metal foam has been increasingly applied in heat exchangers to enhance heat transfer. In this work, the coupled radiation-convection in a rectangular channel filled with metal foam under constant wall temperature is simulated numerically. Discrete-scale approach (DSA) and continuum-scale approach (CSA) are respectively developed for the simulation. In DSA, foam domain is constructed through numerous Kelvin structures, and pore scale field is obtained via CFD simulation with re-radiation flux calculated by surface to surface model; while in CSA, local thermal non-equilibrium model (LTNE) is employed with Monte Carlo method to solve radiative heat transfer, and volume-averaged correlations are determined based on Kelvin structure. The flow field and heat transfer at pore-level generated from DSA are analyzed. By means of a representative volume-averaged post-processing method, thermal behaviors from DSA and CSA are compared in terms of pertinent parameters. It shows that spatial fluctuation phenomenon is observed in pore-level temperature field. The overall heat transfer performance characterized by local Nusselt number is enhanced by inclusion of thermal radiation, and the effect is significant for lower pore density and higher porosity case. Increasing the inlet velocity, pore density or decreasing the porosity all can lead to heat transfer enhancement more or less. For all the cases studied, DSA and CSA can predict the temperature field consistently while the discrepancy between them is relatively larger in the local thermal non-equilibrium region. Additionally, it reveals that CSA is not applicable for accurately predicting energy transport in large gradient heat transfer region due to its volume-averaged characteristic.

Development and validation of a semi-empirical model for two-phase heat transfer from arrays of impinging jets

Two-phase jet impingement is a compact cooling technology that provides high-heat-flux dissipation at manageable pressure drop, with applications in cooling power electronics and server modules. The extensive set of geometrical parameters and operating conditions that determine the heat transfer behavior of jet impingement systems provide an attractive level of design flexibility. In the present study, a semi-empirical approach is developed to predict heat transfer from arrays of jets of liquid that undergoes phase change upon impingement. In the modeling approach developed, the jet array is divided into unit cells centered on each orifice that are assumed to behave identically. Based on prior experimental observations, the impingement surface in each unit cell is divided into two distinct regions: a single-phase heat transfer region directly under the jet, and a surrounding boiling heat transfer region along the periphery. Single-phase convection and boiling heat transfer correlations available in the literature are used to estimate the heat transfer coefficient distribution in each region, and the mean surface temperature of the unit cell is estimated via area-averaging. An analysis is performed to show that the model outputs are sensitive to the heat transfer coefficient correlations used as inputs, with the choice depending on the heat flux input and the expected operating regime. Experiments are performed to validate the area-averaged thermal performance predictions. The model results are also compared against experimental data in the literature. The semi-empirical modeling approach developed in this work successfully represents the different heat transfer modes and transitions that occur during two-phase jet impingement. The location of transition to boiling predicted by the model is consistent with prior experimental observations of an inward-creeping boiling front with increasing heat flux. The predicted temperature difference between the surface and the jet inlet across all experimental comparisons has a mean absolute percentage error of 3.88%. The proposed modeling approach is demonstrated to be a practical tool in the development of two-phase jet array impingement devices, allowing for parametric exploration across the expansive design space.

Refrigeration capacity of silver nanofluids under electrohydrodynamic effect oriented to heat removal in machining process

Nanofluids have been the focus of many laboratories and research centres which seek practical applications for them. Considering the challenges in machining process related with the high temperature in the cutting zone and its influence on the tool life, in this study the nanofluids are employed in a system analogous to the an internally cooled toolholder which can eliminate the use of cutting fluids. In this way, the purpose of this work consists in evaluate the influence of addition of silver nanoparticles in different concentrations in a solution of ethylene glycol/DI-water on the heat transfer coefficient. Moreover, inside the system, the nanofluids were subjected to an electric field in the heat transfer region in order to analyse the influence of the electrohydrodynamic effect. The tests showed that the nanoparticles influenced the fluid viscosity increasing its value up to 20.3% in higher concentrations and also influenced the convective heat transfer coefficient (h). Furthermore, it was presented a positive influence of the electric field enhancing the value of the convective heat transfer coefficient (h) up to 11% when the concentration was 0.039 vol%. On the other hand, the nanoparticle deposition on the heated surface resulted in reduction of the heat transfer coefficient.

Heat and flow analysis of a water droplet on hydrophobic and hydrophilic phase change material

Droplet heat transfer is examined incorporating the phase change material on the hydrophobic surface. Solvent crystallization of polycarbonate wafer is carried out and functionalized silica particles are deposited on the crystallized surface to achieve high water droplet contact angle (125°) with low hysteresis (2°). N-octadecane is used as a phase change material, which is introduced to form a thin layer on the functionalized silica coated surface. Flow and heat transfer analysis are carried out inside the droplet during the melting/solidification of the phase change material. It is found that thin layer deposition of n-octadecane on the treated surface gives rise to reversible hydrophobic-hydrophilic switching surface characteristics. In this case, the state of the treated surface becomes hydrophilic when the phase change material melts on the surface and it recoverably reverses to hydrophobic state when the surface solidifies. Inside the droplet, a single circulation cell is formed during the phase change of n-octadecane coating along the contact line at the droplet bottom. The center of circulation cell moves towards the solid phase side of the phase change material as the phase change progresses along the contact line. The Nusselt number increases with increasing size of the liquid phase on the droplet bottom. The Nusselt number demonstrates two distinguishing regions at the droplet bottom. In the first region, heat is transferred from the liquid phase of the phase change material to the droplet liquid while in the second region heat transfer reverses towards the solid phase of the phase change material, which is associated with the attainment of higher droplet fluid temperature than that of the solid phase change material surface.

Interplay between developing flow length and bubble departure diameter during macroconvection enhanced pool boiling

Enhanced boiling structures based on the concept of separate liquid-vapor (L-V) pathways rely on the motion of the bubbles departing from the nucleating regions (NRs) to induce a macroconvective liquid jet impingement flow over adjacent non-boiling regions. Heat transfer in the non-boiling regions can be improved by incorporating microchannels which act as feeder channels (FCs) that also improve liquid directionality towards the NR. We hypothesize that the single-phase flow characteristics in the developing region of the FC contribute to the boiling enhancement and explore the interplay between the FC length, developing flow length, and departure bubble diameter. FC lengths shorter than the developing flow length benefit from the enhancement due to developing boundary layers over their entire length. However, FC lengths shorter than the departure bubble diameter suffer from bubble interference while FC lengths that are considerably longer than the developing flow length exhibit lower heat transfer rates in the fully developed region. This hypothesis was verified by conducting pool boiling experiments with four feeder channel lengths between 1 mm and 3 mm using HFE-7000, PP1, PP1C, and water. Three distinct regions: (i) interfering bubble, (ii) efficient L-V pathways, and (iii) diminished jet were identified to explain the boiling performance enhancement. This analysis will be beneficial in the pursuit to enhance critical heat flux (CHF) and heat transfer coefficient (HTC) on surfaces utilizing macroconvection mechanisms during boiling with different liquids.

Simulation of dryout phenomenon and transient heat transfer performance of the once-through steam generator based on heat transfer partition

A transient mathematical model is developed based on the distributed-parameter method by dividing the heat transfer regions of the secondary side of the once-through steam generator into single-phase preheating region, subcooled boiling region, nucleate boiling region, liquid deficient region (also known as post-dryout region) and single-phase superheating region in present work. And then it is used to simulate dryout phenomenon and transient heat transfer of the once-through steam generator. The results show that the developed mathematical model and method can be used to reasonably predict dryout phenomenon and transient heat transfer performance at different loads of the once-through steam generator. The heat transfer coefficient of the secondary side decreases sharply when dryout occurs, correspondingly the wall temperature shows a soaring trend and the maximum rising amplitude within present operating range is about 23 °C. The reduction of inlet enthalpy and mass flow rate of the primary side of the once-through steam generator both lead to a reduction of the outlet steam superheat and a downstream movement of the dryout point. The steam at the outlet cannot reach the superheating condition when the inlet enthalpy of the primary side decreases by 5%. The decrease of the inlet enthalpy of the secondary side within current range has little effect on the overall heat transfer performance, while the decrease of the mass flow rate of the secondary side leads to the upstream displacement of the dryout point and a rising outlet steam temperature.

Identification of safe and stable zone of operation in supercritical water reactor

Generation-IV international forum has identified thermal-hydraulic (TH) stability and safety issues as important challenges in the operation of supercritical water reactor (SCWR). In the present work, the scheme of the U. S. SCWR design is analyzed numerically for the TH static and dynamic stability, and safety concerns by employing an in-house TH model which solves nonlinearly coupled mass, axial momentum and energy conservation equations along with the pressure and specific enthalpy dependent thermodynamic equation of state using a characteristics-based implicit time-domain solution methodology to account for the compressibility of the supercritical water (SCW) dynamics. The applicability of the model for the aforementioned purposes has been verified against the state-of-the-art available results. Next, the model is used for the analysis of static instability due to Ledinegg excursion for the U. S. SCWR and results predict the possibility of the instability for the reactor is only at hypothetical operating conditions. The analysis of dynamic instability due to density wave oscillations (DWOs) is performed next. The investigations find its possibility in the relevant operating regime and the stability thresholds are identified to obtain marginal stability boundary (MSBs) for the reactor. Parametric studies are carried out next to determine the effect of exit pressure and coolant mass flow rate on the MSB. Further, the zone of safe operation from metallurgical point of view for the U. S. SCWR at normal and deteriorated heat transfer (DHT) regions is identified after determining the axial variation of heat transfer rate, clad wall temperature and various DHT parameters. Finally, a common stable and safe zone is obtained.

Numerical investigation of spacer effects on heat transfer of supercritical fluid flow in an annular channel

This paper conducts a numerical investigation of spacer effects on heat transfer of supercritical R-134a flows in a vertical annular channel. The simulations are conducted with SST k-ω turbulent model in Fluent 15.0. The investigation range is in the normal and improved heat transfer region at supercritical pressure. The results show that the spacer end enthalpy has remarkably influence on the spacer effects at supercritical pressure, which is different from that at subcritical pressure. In the liquid-like region, the enhancement effectiveness of spacers increases with increasing spacer end bulk enthalpy. However, in the in the gas-like region, the enhancement effectiveness of spacers decreases with increasing spacer end bulk enthalpy. The enhancement effectiveness reaches a peak value near the pseudo-critical enthalpy. For the parameters sensitivity, besides the blockage ratio, spacer end enthalpy and dimensionless distance to the spacer end, the flow parameters and local enthalpy also have significant influences on the spacer effects. Mechanism analysis shows that the characteristic of the mechanism of disruption of boundary layer is the main cause for the appearance of peak value of enhancement effectiveness near the pseudo-critical region at supercritical pressure. The HTC ratio of bare channels can be used to help the prediction of the HTC ratio in the corresponding spacer downstream.

Experimental Study of Subcooled Boiling Heat Transfer of Axial and Swirling Flows inside Mini Annular Gaps

An experimental study of the subcooled boiling heat transfer of axial and swirling upward flows inside vertical mini annular gaps was conducted using deionized water. The subcooled boiling heat transfer coefficients and the boiling curves of the flow inside mini annular gaps with different gap sizes have been investigated. The experimental results both for the single phase heat transfer and subcooled boiling heat transfer inside mini annular gaps showed very good agreement with correlations in the literature. The results showed that the subcooled boiling heat transfer coefficient for a given heat flux increases as the size of the annular gap is decreased. The maximum wall superheat is also influenced negligibly by mass flux. Furthermore, the effects of swirl flow by using spring insets inside the mini annuli on the single phase and subcooled boiling heat transfer have been studied. The results showed that the single phase and subcooled boiling heat transfer coefficients are increased by having swirl flow inside mini annuli using spring inserts. The obtained results also showed that the heat transfer enhancement by having swirl flow inside the annuli using spring inserts decreases as the applied heat flux is increased in the subcooled boiling heat transfer region.

Local Heat Transfer Characteristics of Multi Jet Impingement on High Temperature Plate Surfaces

In order for the hot plate TMCP ultra-fast cooling technology to be optimized, the local heat transfer characteristics of multi jet impinging on hot plate surfaces were investigated. The experiments were performed for double and three jet impingement cooling study as cooling header primary units in the industrial scale. The jet velocity at the nozzle exit ranged between 1.99 m/s and 6.63 m/s. The results demonstrated that both the hydrodynamic structure and the heat transfer region distribution of multi jet impingement cooling were distinct from the single jet case. The parallel flows with a sufficient kinetic energy collided and intensified the heat transfer efficiency in the interference region. The higher-sized nozzle spacing magnified the heat exchange differences in the interference region, whereas the jet velocity increased both the heat flux and the rewetting velocity acceleration outside the stagnation region. The surface temperature in the interference region dropped slightly faster than the parallel flow region at the same spatial distance from the stagnant point, second only to the stagnant point, which was interpreted that the parallel flows interaction and agitation enhanced the heat transfer intensity. The results were valuable in the nozzle arrays arrangement and the heat transfer ability and cooling uniformity improvement of the ultra-fast cooling technology in industrial applications.

The influence of swirling flows on pulverized coal gasifiers using the comprehensive gasification model

The burner swirl intensity plays an important role in membrane wall safety and the gasification performance of pulverized coal, swirl gasifiers. The comprehensive gasification model (CGM), having two reaction regions and one radial heat transfer region, was used to simulate the influence of the swirl number (SN) on gasification performance, multiphase flow, reactions in the space, and reactions regarding the membrane wall. The results show that an SN of 0.66 is a critical point to divide low-swirl and high-swirl flows for the single-swirl-burner gasifiers of this study. Gasifier performance increases sharply with increased SN for low-swirl burners, whereas SN has little influence on gasifier performance (and multiphase flow) for high-swirl cases.In this paper, we examine the relationship between burner swirl and flow fields, temperature distributions in the space and on membrane wall, slag thickness, and the molten slag velocity distribution on the membrane wall. The influence of SN on these parameters is more remarkable at the top of the reattachment point than below it. The slag layer formed on the membrane wall isolates high-temperature gas effectively, except around the reattachment point. To guarantee gasifier performance and membrane wall safety, the burner SN should be between 0.66 and 0.9 for the single-swirl-burner gasifiers of this study.

Investigation of permeability and porosity effects on the slip velocity and convection heat transfer rate of Fe3O4/water nanofluid flow in a microchannel while its lower half filled by a porous medium

In this study, we numerically investigated the forced heat-transfer and laminar flow in a two-dimensional microchannel whose lower half was filled with a porous medium. The nanoparticles used were Fe3O4 and a water-based fluid. The nanoparticles were considered in the form of a completely stable suspension in a water-based fluid. The nanofluid flow in this microchannel was modeled employing the Darcy–Forchheimer equation. We also hypothesized that there was a thermal equilibrium between the solid phase and nanofluid for energy transfer. And the walls of the microchannels were assumed at a constant temperature higher than the inlet fluid temperature. Also, the slip boundary condition was assumed along the walls. The effects of Darcy number, porosity and slip coefficients, and Reynolds number on the velocity and temperature profiles, and local Nusselt number were studied in both porous and non-porous regions in this research. In this study, the Darcy number was assumed to be Da = 0.1 and 0.01, Reynolds number Re = 25, 50, and 100, slip coefficient B = 0.1, 0.01, and 0.001, the porosity of the porous medium ε = 0.5 and 0.9, and the volume percentage of the nanoparticles φ = 0%, 2%, and 4%. With the Darcy number decreasing, the local Nusselt number increased in the non-porous region, and decreased in the porous region. And this phenomenon was observed for the first time. The increase in the Reynolds number increased the heat transfer in both regions. For instance, the local Nusselt number increased 4 times with the Reynolds number changing from 25 to 100 under the same conditions. The decreased Darcy number in the porous medium increased the amount of slip velocity near the walls in the non-porous region, and on the other hand, the decreased Darcy number in the porous medium reduced the slip velocity in the porous region. Also, the jump observed in the slip velocity, was due to the presence of the fluid velocity in the microchannel width.

Vane Suction Surface Heat Transfer in Regions of Secondary Flows: The Influence of Turbulence Level, Reynolds Number, and the Endwall Boundary Condition

Secondary flows in vane passages sweep off the endwall and onto the suction surface at a location typically close to the throat. These endwall/vane junction flows often have an immediate impact on heat transfer in this region and also move any film cooling off the affected region of the vane. The present paper documents the impact of secondary flows on suction surface heat transfer acquired over a range of turbulence levels (0.7–17.4%) and a range of exit chord Reynolds numbers (500,000–2,000,000). Heat transfer data are acquired with both an unheated endwall boundary condition and a heated endwall boundary condition. The vane design includes an aft loaded suction surface and a large leading edge diameter. The unheated endwall boundary condition produces initially very high heat transfer levels due to the thin thermal boundary layer starting at the edge of heating. This unheated starting length effect quickly falls off with the thermal boundary layer growth as the secondary flow sweeps up onto the vane suction surface. The heat transfer visualization for the heated endwall condition shows no initial high heat transfer level near the edge of heating on the vane. The heat transfer level in the region affected by the secondary flows is largely uniform, except for a notable depression in the affected region. This heat transfer depression is believed due to an upwash region generated above the separation line of the passage vortex, likely in conjunction with the counter rotating suction leg of the horseshoe vortex. The extent and definition of the secondary flow-affected region on the suction surface are clearly evident at lower Reynolds numbers and lower turbulence levels when the suction surface flow is largely laminar. The heat transfer in the plateau region has a magnitude similar to a turbulent boundary layer. However, the location and extent of this secondary flow-affected region are less perceptible at higher turbulence levels where transitional or turbulent flow is present. Also, aggressive mixing at higher turbulence levels serves to smooth out discernable differences in the heat transfer due to the secondary flows.