Region Heat Transfer Research Articles

Numerical Investigation of Heat Transfer Characteristics of Pin-Fins with Roughed Endwalls in Gas Turbine Blade Internal Cooling Channels

In modern jet engines, the effective cooling structure design is essential to both increase turbine efficiency and, in the meantime, maintain structural integrality. Pin-fin arrays play a crucial role in the cooling mechanism of the turbine blade internal cooling system. In the context of investigating the efficiency of cooling techniques using pin-fins, while the other studies focus on the pin-fin configurations, the current study is a step towards optimizing cooling cascade endwalls for better maneuver and reservation of vortices which lead to higher heat transfer near the endwalls. This study presents the results from the investigations of the flow-field and heat transfer characteristics of the pin-fin arrays with roughed endwalls, i.e., recessed endwalls and extruded endwalls types. The heat transfer and pressure drop characteristics of the channel are numerically examined to compare with those of the flat endwalls case within a range of inlet Reynolds number, which varies from 7400 to 36000. Both the leading and trailing sides of the channel are divided into five regions in the streamwise direction to understand further the heat transfer capacity of the pin-fins and the endwalls. The results show that, with these new endwall configurations, the high heat transfer regions near the pin-fins are remarkably enlarged compared to the flat endwalls. And in the meantime, both the new endwall arrangements enhance the heat transfer capacity of the channel near the pin-fins. The two new designs outperform the baseline case, with the heat transfer efficiency index (HTEI) improved by 37.8% for the recessed endwalls and 15.9% for the extruded endwalls. By varying the height of the indentation and the protrusion, it is found that, with smaller heights, both configurations produce a much lower friction factor. However, with much higher heat transfer capacity, leading to relatively higher values of heat transfer efficiency index (HTEI), up to 77.7% and 41.5% of the recessed and extruded endwalls are higher than HTEI of the channel with flat endwalls. These results indicate the great potential for improving the heat transfer capability of pin-fins by optimizing endwall configurations.

Experimental study on thermal and friction characteristics of liquid flow across a heating cylinders under low-frequency ultrasound

• The heat transfer of water flow across a cylinder under a 25 kHz ultrasound was studied. • Visualization by TLCs on shaded surface was achieved using unique image processing. • Heat transfer area on cylindrical surface could be identified by TLCs. • The waves emitted downstream could affect the cylindrical surface at the upstream. • A prediction formula for Nu of the flow across a cylinder under ultrasound was proposed. The heat transfer capability was investigated for water flow across a cylinder under the influence of 25 kHz ultrasound in a rectangular duct with a Reynolds number ( Re ) in the range of 110–2,140. A heating cylinder with constant heat flux was set at the center of the duct. The transducer was mounted on the top wall, and its location was set at -2, -1, 0, +1, and +2 diameters from the cylinder in the streamwise direction. Thermocouples and thermochromic liquid crystals (TLCs) were used to characterize the heat transfer mechanism on the heating surface. In particular, a visualization technique using TLCs was developed to obtain temperature results on a cylindrical surface. The temperature of the heating surface was reduced by the acoustic cavitation process with ultrasonic involvement. The local Nusselt number was increased by 2.37-fold by the waves at a cylindrical angle of 90° and Re value of 110 when the transducer was also located in the middle of the duct. Furthermore, the results indicated that ultrasonic effects could be transported along with the flow when the transducer was located upstream. In addition, when the waves were released downstream, they affected the heat transfer of the cylindrical surface at low Re values. The friction loss in the system was investigated by comparing the pressure drops with and without ultrasound. Depending on the position of the ultrasonic transducer, the pressure drop ratio was in the range 0.88–1.27. Furthermore, the areas most affected by ultrasound were identified using the TLC measurement method, and regions of augmented heat transfer were detected locally on the shaded and unshaded sides of the cylinder. A predictive formula was proposed for the local Nusselt number ratio with and without ultrasound as a function of the Re value, spanwise direction, and cylindrical angle.

Modeling and analysis of steam-water two-phase flow distribution and wall temperature distribution in parallel heated pipes with different manifold types

In solar tower receiver, boiler water-wall and various heat exchangers, the uneven distribution of two-phase flow often occurs among parallel heated pipes, which seriously reduces the operating efficiency and may further induce heat transfer deterioration and soaring wall temperature. In this paper, a coupling model for the flow distribution and heat transfer process of steam-water two-phase flow in parallel heated pipes was established based on the discrete methods. In the present model, two main improvements were proposed as below. On one hand, the calculated equations for the pressure drop and the phase distribution of two phase fluid in both the combining T-junction and dividing T-junction were established to consider the coupling effects of the inlet manifold and outlet manifold on the flow distribution characteristics. On the other hand, the calculation module for the heat transfer and wall temperature along the branch pipes was also coupled in the present model with considering of different heat transfer regions (saturated nucleate boiling region, liquid deficient region, and superheated steam region) along the pipe. The present model was then verified and used to investigate the effects of system pressure, inlet fluid flow, inlet steam quality and manifold type on the flow distribution characteristics of high temperature and high pressure steam-water in parallel vertical upward pipes, and the effect of the uneven distribution of the gas-liquid two phases on the system safety was analyzed and evaluated. It was found that gas phase preferentially enters the first branch pipe near the inlet of the dividing manifold affected by buoyancy, and this results that the inlet steam quality of the corresponding branch pipe is always obviously higher than that of the subsequent pipes. As the system pressure decreases, the outlet fluid enthalpy distribution and outlet wall temperature distribution among branch pipes become more uneven. Moreover, the outlet fluid enthalpy and outlet wall temperature of the branch pipes closer to the inlet of the dividing manifold increase sharply with the decrease of system pressure. In the U-type parallel pipe system, the branch pipes away from the inlet of the dividing manifold are more prone to wall overheating at the outlet, while the opposite is true for the Z-type parallel pipe system.

Influence of surface curvature and jet-to-surface spacing on heat transfer of impingement cooled turbine leading edge with crossflow and dimple

The impingement cooling is used to protect turbine components under high thermal load. This paper conducts a numerical study of heat transfer characteristics of turbine leading edge with lamilloy cooling structure. Target surface curvature and jet-to-surface spacing under various crossflow Reynolds number conditions are considered. Meanwhile, the target surface is dimpled to enhance heat transfer capability. Besides, staggered film holes are considered in the computational domain. The results show that the crossflow induces a counter rotating vortex pair that greatly increase the near wall turbulence and local heat transfer. The vortex pair forms the bound for a U-shaped high heat transfer region caused by the jet impingement. The target surface curvature tends to enlarge the relative area of the U-shaped region. The increased crossflow strength weakens the jet cooling effect, the total heat transfer performance is therefore decreased. Large jet-to-surface spacing enhances the impingement cooling when crossflow is weak but the total heat transfer will not recover as crossflow Reynolds number get higher compared with small and medium spacing cases.

Universal approach to predicting full-range post-dryout heat transfer under uniform and non-uniform axial heat fluxes

In the present study, based on three experiments including two uniform axial heat flux experiments (Becker and KIT) and one non-uniform axial heat flux experiment (BeckerII), an universal mechanistic model was developed for the full-range post-dryout heat transfer. The whole post-dryout heat transfer region consists of a developing region and a fully developed region. A determination equation for the length of the developing post-dryout region was proposed and verified to distinguish and quantify these two regions, which can be used to calculate the peak wall temperature position in most cases. The mechanistic model considers three-path heat transfer that involves heat transfer from heated wall to vapor, from vapor to droplets, and from direct contact between wall and droplets. The cross-section of the flow was separately considered in the modeling as a film region and a core region, which makes it possible to account for the impact of the droplets’ concentration distribution over the cross-section on the interfacial heat transfer between vapor and droplets. Model assessments cover a broad range of flow conditions, around 25,000 data points, and multiple types of fluid. Results indicate that the proposed model could provide precise predictions for the full-range post-dryout heat transfer.

Experimental study on the augmented Nusselt number of the endwall through a square-sectioned sharp-turn channel using novel heat exchanger

Aquiring higher thermal efficiency with minimum energy consumption is the continuous pursuit of heat exchanger design. This work aims to demonstrate a novel enhanced heat exchange approach for a two-pass heat exchanger surface using four forms of a novel heat exchanger (nHEX), such as high straight and low curved turbulators, etc. The nHEXs are arranged parallel and equally spaced on the internal endwall. A comparison between current design and conventionally turbulated endwalls of sharp-turn channels is presented. The results indicate that the heat transfer rate is greatly augmented by inserting an array of nHEXs compared to a smooth endwall channel. The strategically placed vortex generators can improve the lowest heat transfer region on the smooth endwall to achieve overall thermal enhancement and temperature uniformity. Among the studied geometries, higher height or straight forms of the nHEXs are prone to enhance endwall heat transfer. The high straight forms of the nHEXs provide the maximum improvement of the normalized Nusselt number and thermal enhancement factor in a sharp-turn channel, which are 32% and 18% higher than the corresponding values of the smooth endwall, respectively. This study proved the potential of an array of parallel nHEXs to provide endwall heat transfer enhancement in a sharp-turn channel with an acceptable pressure drop penalty.

High thermal conductivity of stishovite promotes rapid warming of a sinking slab in Earth's mantle

Thermal transport in subducted slabs and mantle critically influences their thermo-chemical evolution and dynamics, where the thermal conductivity controls the magnitude of conductive heat transfer. Here we investigate high-pressure thermal conductivities of stishovite and new-hexagonal-aluminous (NAL) phase, two major slab minerals in the shallow lower mantle, and their impacts on slab dynamics. Pure and Al-bearing stishovite exhibit conductivities of 60–70 and 30–40 Wm−1K−1, respectively, at 25–60 GPa and 300 K, much higher than the Fe-bearing NAL, 10–33 Wm−1K−1, and assemblage of a pyrolitic mantle or subducted basaltic crust. Numerical simulations indicate that subducted crustal materials particularly with local silica-enrichment would have efficient thermal conduction which promotes faster warming of a sinking slab, altering dynamic stability of slab materials and leading to slab stagnation and crust detachment in the shallow lower mantle. Conductive heat transfer in silica-enriched regions along subducted slabs in the shallow lower mantle can be more influential on mantle dynamics than previously thought.

Experimental study of condensation heat transfer of R134a inside the micro-fin tubes at high mass flux

The flow condensation heat transfer coefficient of R134a inside two helical micro-fin tubes with outer diameter of 6.35 mm and helical angles of 18o and 28o respectively were experimentally studied at a high mass flux ranging from 500 to 1100 kg m−2 s−1. The condensation temperatures are 35°C, 40°C and 45°C, and the vapor qualities ranges from 0.95 to 1.00 at the inlet section and from 0.00 to 0.05 at the outlet section. Experimental results show that there are annular and intermittent flow regimes inside the tube under experimental conditions, the ratio of annular flow region to the total heat transfer region is about 70%. The liquid film flow changes from rotational flow to coexistence of rotational and horizontal flows when the vapor quality is 0.475±0.08, 0.486±0.06 and 0.521±0.05, respectively and the corresponding saturation temperature is 35°C, 40°C and 45°C. Based on the experimental data, the heat transfer coefficient increases with increasing mass flux and fin helical angle, and with decreasing saturation temperature and Reynolds number of cooling water. In addition, the heat transfer coefficient of annular flow is greater than that of intermittent flow, in other words, the heat transfer coefficient increases with increasing vapor quality. The heat transfer coefficient obtained at high mass flux was compared with the calculated data of some published correlations used at low mass flux. It was found that most correlations underestimate the experimental heat transfer coefficient inside the micro-fin tube, and these prediction deviation increases with increasing mass flux and fin helical angle. By introducing some dimensionless numbers as well as considering the flow mechanism of liquid film inside the tube, two heat transfer coefficient correlations were proposed based on the experimental data. The ratio of liquid film thickness to fin height was used to characterize whether the liquid film flow is rotational flow or coexistence of rotational and horizontal flows in the proposed correlation, which shows a better prediction accuracy with a mean deviation of less than 7.3%.

Accuracy improvement of the transient heater foil technique for heat transfer tests: preliminary results

Experimental heat transfer measurements are used in a wide range of fields, for example to validate new cooling concepts in turbomachinery, to assess the performances of heat exchangers, and to provide data for numerical simulations. A particular challenge is posed by complex geometries, where the heat transfer coefficients cannot be determined with the usual transient heater mesh method. One way to address these complex systems is the transient heater foil method, which generates a constant heat flux in the metal foil, which is attached at the surface to be measured. However, the accuracy of the measurement remains an open issue compared to the heater mesh method. Here we show a modification of the heater foil method, which uses a linearly increasing heat flux in the foil to improve the measurement accuracy, especially in low heat transfer regions. The new method is presented using a single impingement cooling setup; results demonstrate good agreement with the baseline method (heater foil with step heating) and the literature, while the accuracy is improved.

Effects of the casing step size on the tip leakage flow pattern and heat transfer characteristic of turbine blade squealer tip

In this paper, under the GE-E3 turbine stage condition, the effect of the casing step on the heat transfer performance is obtained by solving three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations. The heat transfer characteristics predicted by standard k-ω model are consistent with the experimental data. The simulation results reveal that, the tip heat transfer coefficient decreases and the area of the high heat transfer region is reduced by introducing the casing step structure. Compare to the baseline case, when the casing step height is 1%, 2% and 3% of the blade height, the area-average heat transfer coefficient at the blade tip decreases by 6.1%, 6.7% and 7.0%, respectively. The introduction of casing step structure can eliminate the swirling strength of Scraping Vortex (SV), thus weaken the reattachment of tip leakage flow on the cavity bottom. Under the step vortex effect, the mainstream from the tip pressure side entering the tip cavity is reduced and replaced by entering from the tip suction side. Because of the lower pressure difference between the tip suction side and the tip clearance, the swirling strength of SV is decreased. Arranging a casing step with the same size as the tip clearance upstream the blade can reduce the tip heat transfer performance to the greatest level. The introduction of casing step structure also has a negative effect on the output power and efficiency of the turbine, it is necessary to balance the aerodynamic and heat transfer of the turbine in order to achieve the optimal results.

Mathematical modeling for bubbling fluidized bed CO-methanation reactor incorporating the effect of circulation and particle flows

Fluidized bed reactor is promising for CO methanation owing to its excellent heat transfer performance. The gas flow distribution between bubble and emulsion phases, and the characteristics of heat removal are significant for such a solid-catalyzed exothermal reaction in fluidized bed but these are described simplistically in most conventional models. In this work, a two-phase model contemplating: (i) the effect of circulation flow on gas flow distribution, (ii) the effect of particle flow on reaction heat removal, was proposed. The proposed model was validated by comparison with experimental data of the fluidized bed CO-methanation reactor. Meanwhile, simulation results of the proposed model and Kunii–Levenspiel model were compared. Finally, sensitivity analysis of important parameters was carried out. The results indicate that: (i) particle flow has more significance than circulation flow; (ii) the effect of bubble size is limited within bubbling region and bed-wall heat transfer has obvious influence in freeboard.

Effects of rock-airflow conjugated heat transfer in development headings: A numerical study

Excavation of an airway in an environment with high strata heat loads is inevitable in deep mining engineering. To protect the health of miners and ensure operational efficiency, it is important to control the thermal environment in the heading faces based on the accurate calculation of the cooling loads. This study aimed to reveal the rock-airflow heat-transfer mechanism in headings with a forced auxiliary ventilation system. A computational fluid dynamics numerical simulation was applied to study the effects of factors, such as the cross-sectional shape of the heading, roughness on the rock surfaces, and Reynolds number of the airflow, on the rock-airflow heat-transfer phenomenon. These were represented by the responses of the distributions of the convective heat-transfer coefficient, wall temperature, and longitudinal length of the heat-transfer region. The effects of the above-mentioned factors on the response indices were investigated using an orthogonal experimental design. The study showed that the Reynolds number had the highest influence on the convective heat-transfer coefficients and wall temperatures, while the cross-sectional shape of the heading had the most significant influence on the longitudinal lengths of the heat-transfer region. Thus, this study provides an important basis for determining the cooling strategy in development headings.

Transient liquid crystal thermography using a time varying surface heat flux

Heat transfer measurements are required in a wide range of fields, for example to validate new cooling concepts in turbomachinery, to assess the performances of heat exchangers, and to provide data for numerical simulations. Thereby transient methods are often applied for local heat transfer resolution. A particular challenge is posed by complex flows, where the determination of the heat transfer coefficients with the commonly applied transient heater mesh method can prove difficult, for instance in cases in which the flow can take different paths leading to mixing flows at different temperatures, and a difficult determination of the reference temperature. One way to address these complex systems is the transient heater foil method, in which the experiment is driven by a constant heat flux generated at the surface under study, instead of a temperature variation in the flow. However, the accuracy of the measurement remains an open issue compared to the heater mesh method. Here we show a modification of the heater foil method, which uses a linearly increasing surface heat flux to improve the measurement accuracy, especially in the low heat transfer regions. The new method is validated by measuring the heat transfer of a single circular jet perpendicularly impinging on a flat plate, and by comparing the results to a correlation available in the literature. Results show good agreement with the literature, while providing considerable accuracy improvement with respect to the heater foil method with constant heat flux. The heater foil method presented here, reaches similar uncertainty values as the state of the art versions of the heater mesh method in low heat transfer regions, while providing better accuracy in the high heat transfer regions. Additionally, it allows for an easier implementation for certain problems, provided that optical access is guaranteed and the surface curvature allows for the addition of the heater foil.

Experimental research on film cooling characteristics of multi-film-hole-rows

The present study experimentally explores the effect of the film-hole-row layout and hole angle on the film cooling characteristics of the multi-rows of micro-film-hole by the transient infrared technique. Reynolds number based on the diameter of the micro film hole and Knudsen number are 7000 and 7 × 10−5, respectively, and the dimensionless parameters of the experiment are the same as those of the real turbine blade. Two blowing ratios of 0.5 and 1.0 are investigated. Five rows of the film hole in leading edge have a pronouncedly positive effect on the film cooling effectiveness in the upstream region, and obtain an excellent coolant coverage at the blowing ratio of 1.0. The Nusselt number of the whole test surface is significantly decreased by five rows of the film hole in leading edge with the hole angle of 45° at the blowing ratio of 1.0. Increasing the hole angle from 45° to 90° decreases the span-wise averaged film cooling effectiveness of the test surface, and this phenomenon is obvious at the blowing ratio of 1.0, the maximum is about 29 %. There is a high heat transfer region at the beginning of the test surface of the five rows of the film hole in leading edge with the hole angle of 90° structure due to the mainstream-blockage-effect.

Modeling of Reversible Solid Oxide Cell Stacks with an Open-Source Library

Fuel cells are potential contenders of the traditional internal combustion engines, with relative high efficiency and low/no emission of pollution and greenhouse gases. Electrolyzers convert electricity into chemical energy, e.g. hydrogen, and in case of high-temperature electrolyzers in syngas, e.g. hydrogen and carbon-monoxide, etc. A reversible solid oxide cell (rSOC) is able to operate in both modes. This represents a promising electrochemical device for future applications. Comprehensive experimental investigations on rSOCs are typically expensive and time-consuming. These may be conveniently augmented by numerical models to improve and optimize and enable the provision of more rapid prototypes. These models usually consider some or all of the underlying physical processes, e.g. heat and mass transfer (including thermal radiation), structural analysis, etc. They range from zero-dimensional (0-D) and 1-D models in system-level models1, 2 to coupled 3-D models in cell/stack-level applications.3–5 Stack/system level models are attracting more and more attention due to their practical applications to commercial/industrial designs. In this work, a steady-state, 3-D, non-isothermal, and homogenized stack model has been developed from an extension of previous studies4–6 and implemented into the open-source library, OpenFOAM.7 It addresses all major physical processes, including fluid flow, heat and mass transfer (including thermal radiation), and electrochemical reactions. The model strategy applies a multi-region approach to consider every unit-cell as different sub-regions, namely, cathode, anode, electrolyte, and interconnect. Inter region heat and mass transfer is prescribed during the calculations. The micro-structures within the subregions are treated using volume-averaging method, in which case, the flow distribution can be easily described by replacing the viscous losses with a drag or friction term in the governing momentum equation(s). This greatly decreases the computational effort,6 and predicts quite similar results to those from experimental measurements.5 In the present work a surface-to-surface (S2S) model, also referred to as a viewFactor model, is utilized to simulate the external thermal radiations. Internal radiation is neglected due to its relative unimportance.Figure 1: Comparisons of temperature variations between experimental measurements and simulations, i = 0.5 A cm−2 . The present model is applied to investigate the performance of an in-house designed rSOC stack, the key component of a 5/15kW-class rSOC system.8 A 1-D simulation is also performed with a previous developed Simulink model.1 The stack consists of four 10-layer sub-stacks with an active cell area per layer of 320 cm2. It operates in both fuel cell and electrolysis modes, in a temperature range of 600 – 800 ◦C. To reduce heat losses to adjacent components and ambient environment, it is surrounded by insulating plates. More details can be found in the recent publication.8 As shown by Figure 1, temperature variations are compared between experimental measurements and numerical predictions, at a mean current density of 0.5 A cm-2 . In fuel cell mode, the cell voltages are 0.818 V, 0.821 V, and 0.824 V, for the experiment, Simulink, and stack model, respectively. In the electrolysis mode, the values are 1.240 V, 1.234 V, and 1.260 V, respectively. These results indicate fairly good agreement between the predictions and experiments. It can be found that the stack model predicts smaller temperature deviations than is seen in the experiments and the Simulink predictions. However, the former consumes approximately 1 hour for every run, while the later costs only several minutes. In future work, thermal stresses, which play important roles in design optimization, will be considered. Considering the 3-D nature of the stress distribution, the coupling between present model and a structural analysis program/solver would be a promising topic.

Numerical Predictions of Turbine Cascade Secondary Flows and Heat Transfer With Inflow Turbulence

Abstract Turbine passage secondary flows are studied for a large rounded leading edge airfoil geometry considered in the experimental investigation of Varty et al. (“Vane Suction Surface Heat Transfer in Regions of Secondary Flows: The Influence of Turbulence Level, Reynolds Number, and the Endwall Boundary Condition,” ASME J. Turbomach., 140(2), p. 021010) using high resolution large eddy simulation. The complex nature of secondary flow formation and evolution are affected by the approach boundary layer characteristics, components of pressure gradients tangent and normal to the passage flow, surface curvature, and inflow turbulence. This paper presents a detailed description of the secondary flows and heat transfer in a linear vane cascade at exit chord Reynolds number of 5 × 105 at low and high inflow turbulence. Initial flow turning at the leading edge of the inlet boundary layer leads to a pair of counter-rotating flow circulation in each half of the cross plane that drives the evolution of the pressure side and suction side of the near-wall vortices such as the horseshoe and leading edge corner vortex. The passage vortex for the current large leading edge vane is formed by the amplification of the initially formed circulation closer to the pressure side (PPC) which strengthens and merges with other vortex systems while moving towards the suction side. The predicted suction surface heat transfer shows good agreement with the measurements and properly captures the augmented heat transfer due to the formation and lateral spreading of the secondary flows towards the vane midspan downstream of the vane passage. Effects of various components of the secondary flows on the endwall and vane heat transfer are discussed in detail.

A new heat transfer correlation for supercritical water flowing in a vertical tube - an hitherto approach

This paper provides an hitherto new approach of a heat transfer correlation based on heat flux to mass flux ratio for predicting wall temperature of supercritical water flowing in a bare vertical tube. Totally 10,174 numbers of experimental data were collected, Out of which 4236 data are heat flux/mass flux ≤0.4 and 5938 data are heat flux/mass flux>0.4. These data were obtained from various experimental resources and Lookup table and it covers q/G ranges from 0.085 to 1.295, mass flux ranges from 201 to 3500 kgm−2 s−1, heat flux ranges from 129 to 1600 kWm−2, pressure ranges from 225 to 320 bar, diameter of the tube 4.4 to 38 mm and bulk fluid enthalpy ranges of 250 to 3200 kJkg−1 of vertical tube working at various supercritical working conditions based on normal, enhancement and deteriorated heat transfer regions. By using these data, a new heat transfer correlation is derived for finding the wall temperature by Levenberg-Marquardt (LM) algorithm of multi dimensionless analysis method with Nonlinear curve fitting method. A general form of forced convection heat transfer correlation was obtained by Buckingham Π theorem. The Wall temperatures predicted by various 7 existing correlations along with this new correlation are found by Visual basic program and compared with experimental data for evaluating the deviations. The rank was provided based on error band from ±0.5% to ±50% etc., and error analysis were conducted for proving the best fitting correlation.

Study on flow and heat transfer performance of X-type truss array cooling channel

In this study, an innovative cooling channel filled with a novel X-type truss array structure was proposed to improve the cooling effect of turbine blade mid-chord region. The flow and heat transfer characteristics of the X-type truss array cooling channel were analyzed numerically. The influences of Reynolds number (ReH = 10,000–60,000), truss rod diameter ratio (d/H = 0.1–0.3), transverse spacing ratio (Xs/C = 1-3) and streamwise spacing ratio (Zs/C = 1-3) on the cooling performance of X-type truss array channel were investigated. Then the empirical correlations of heat transfer coefficient and friction factor were obtained by nonlinear regression method for the X-type truss array cooling channel. The results show that horseshoe vortexes form around each truss rod of truss elements. The high heat transfer regions mainly appear in the downstream of each truss rod on the channel wall as well as the windward side of truss rod surface. Increasing ReH can effectively enhance the heat transfer coefficient of the X-type truss array cooling channel, but has little influence on the friction factor of the channel. When d/H increases from 0.1 to 0.3, the friction factors of the X-type truss array channel increases about 3.5–4.4 times, and the average Nusselt numbers of the channel increase by 57.5%–87.9%. When Xs/C increases from 1 to 3, the friction factors and the average Nusselt numbers of the channel decrease by 83.9%–85.1% and 44.9%–57.7% respectively. The friction factor and average Nusselt number of X-type truss array cooling channel both first increase and then decrease with the increase of Zs/C. The moderate values of d/H, Xs/C and Zs/C can make the comprehensive thermal performance of X-type truss array cooling channel better. The results may provide a guidance and reference for the application of X-type truss array structure in the internal cooling channel of advanced gas turbine blades.

Investigation of heat transfer and flow properties in separated flow and reattachment regions for liquid sodium flow at fast reactors

In many engineering applications, such as heating and cooling systems, flow around an automobile or building, separation and reattachment regions are formed. These regions are very important for controlling the amount of heat and mass transfer. Liquid sodium is used as heat transfer fluid in many areas having a high temperature, especially nuclear reactors. In this work, the effects of flow separation and reattachment on the heat transfer and flow properties of step corner structures with different chamfer lengths as h/4, h/2, 3 h/4 and h comparing with and without chamfer (normal) geometry have been numerically investigated in the vertically positioned backward facing step flow geometry for fast reactors that need to be cooled efficiently. One of the walls behind the backward facing step has a constant temperature while the others are adiabatic. The results of the work have been determined by solving steady conservation equations with three dimensional and k-ε turbulence model with Boussinesq approach using ANSYS-FLUENT computer program. Water and liquid sodium have been employed as working fluids. The expansion rate of the backward-facing step is 1.5. The study has been carried out at two different Reynolds numbers: 5000 and 10000. The presented work has been compared with the numerical results of the study found in the literature and it has been seen that they are compatible and acceptable with each other. The results have been exhibited as the variations of Nu number, fluid temperature, turbulence kinetic energy and pressure. Besides, temperature, velocity and streamline distributions have been visualized in backward facing step flow geometry. For Re = 10000, it has been determined that average fluid temperature increment value of the backward facing step geometry with h chamfer length in the liquid sodium flow has been 6.54 K higher than in water.

Effect of Entrance Geometry on Heat Transfer in a Rib-Roughened Rectangular Channel

The effect of the entrance geometry conditions on the local and average heat transfer coefficient in a rectangular channel with 45° angled ribs has been investigated. Ribs were deployed on one side only, the remaining walls of the channel being assumed smooth and unheated. The influence of entrance conditions was addressed by considering either a long ribbed unheated section or a smooth channel upstream of the ribbed heated section. As known, channels with 45° angled ribs are characterized by spanwise variations of heat transfer coefficient due to secondary flows, with high heat transfer levels close to one side and low heat transfer levels on the opposite side. Numerical results, checked against experimental data, showed that heat transfer coefficients of first modules are affected by entrance conditions, especially in the most effective heat transfer region. Regardless of entrance condition, a quasi-periodically fully developed heat transfer condition is attained only in the low heat transfer region, while elsewhere heat transfer coefficient distributions are periodic in shape but values tend to increase with the streamwise coordinate. These findings indicate that heat transfer experiments on ribbed channels should be generally accompanied by a careful description of the entrance conditions assumed in the experiments.