Endwall heat transfer and pressure drop in rectangular channels with attached and detached circular pin-fin array

Endwall heat transfer and pressure drop characteristics in four rectangular channels with a channel aspect ratio of 4 and the staggered arrays of circular pin-fins with four clearances (C) between pin-tips and the measured endwall of 0, 1/4, 1/2 and 3/4 pin-diameter (d) are examined comparatively at Reynolds numbers (Re) of 10,000, 15,000, 20,000, 25,000 and 30,000 to determine the effects of pin-tip leakages on the endwall heat transfer and on channel inlet-to-exit pressure drops. The accelerated flows through pin-to-endwall clearances modify the protrusion-endwall interactions that affect the horseshoe vortices as well as the downstream wakes and shear layer separations. By way of increasing C/d ratio from 0 to 3/4, the area averaged endwall Nusselt numbers decrease with substantial reductions in channel inlet-to-exit pressure drops. The endwall heat transfer level with detached pin-fins at C/d = 1/4 is somewhat less than that with attached pin-fins but the pressure drop coefficient of the former is much lower than that of the later, which leads to the highest thermal performance factor among the four comparative cases in the Re range examined by this study. A set of correlations that evaluate the area averaged endwall Nusselt number and the pressure drop coefficient with Re and C/d as the controlling parameters are derived.     

Influence of Tube Arrangement on the Thermal Hydraulic Performance of a Membrane Helical-Coil Heat Exchanger

Thermalhydraulic performances of membrane helical-coil heat exchangers in in-line and staggered arrangements, are numerically investigated. The influences of Reynolds number, dimensionless pitch, and arrangement on heat transfer and flow are discussed. Moreover, the axial and tangential velocity distributions were presented for various pitches and arrangements. The results show that the membrane helical-coil arrangement has a profound effect on the thermalhydraulic performance. For the smaller radial pitch (s ₁/d < 1.8), the heat transfer coefficient in in-line arrangement is higher than that in staggered arrangement. Meanwhile, the in-line arrangement gives a significantly higher friction factor than the staggered arrangement. However, for the larger radial pitch (s ₁/d ≥ 1.8), the coil arrangement appears to have no effect on the heat transfer and friction factor. In addition, the average Nusselt number Nu and the friction factor f for in-line and staggered arrangements were calculated and correlated against the Reynolds number and structural parameters. According to the thermalhydraulic performance evaluation criterion, the staggered arrangement is recommended as the optimal coil arrangement.     

Endwall heat transfer and pressure drop in scale-roughened pin-fin channels

There is a growing requirement for improved heat transfer performance for a number of electronic devices and this dictates a need to further elevate the endwall heat transfer performances for pin-fin channels. Driven by this need, a novel compound heat transfer enhancement (HTE) measure that combines deepened scales and pin-fin array is devised. Characteristics of heat transfer and pressure drop performances in two scale-roughened pin-fin channels with two different pin pitch-to-diameter ratios are compared for both forward and backward flows in the Reynolds Number (Re) range of 1000–30000. Comparisons of heat transfer data, pressure drop measurements and thermal performance factors with previous results collected from a variety of single and compound HTE devices demonstrate the significant augmentations in both heat transfer rates and pressure drop coefficients for the present HTE measure. This present compound HTE measure with scales and pin-fin array demonstrates an enhancement on the heat transfer up to of 22 times of the developed flow references in smooth-walled pipe within the Re range of 1000–30000. Experimental correlations of heat transfer and pressure-drop coefficients for two scale-roughened pin-fin channels with forward and backward flows are derived to assist design applications.     

Surface temperature reduction by using dimples/protrusions in a realistic turbine blade trailing edge

Abstract

In this article, numerical simulations have been conducted on the heat transfer effect of dimple/protrusion layouts of a pin-finned wedge duct. Conjugate heat transfer calculations are further performed to investigate the cooling effect of modified schemes with dimples and protrusions added. Comparisons are carried out with a turbine second stage guide vane employed as the prototype. The dimple/protrusion-pin fin arrangement is set as the optimum one obtained above, and dimple depth/protrusion height varies from 0.2 to 0.3 times the structure diameter. It is found that the side-by-side arrangement and protrusion structure is more beneficial for the wedge duct endwall heat transfer. Comparison with the prototype blade shows that the addition of both dimples and protrusions are helpful in enhancing the trailing edge cooling effect. The cooling effect is increased with an increase in dimple depth/protrusion height. The results also show that the modified blade with protrusions attached at 0.3 height saves 0.48?g/s cooling mass flow and reaches the most positive performance with a 17?K, 14?K average temperature reduction, 0.022, 0.018 cooling effect increasing for pressure, suction side, respectively.     

An experimental investigation of heat transfer in pin fin array

Detailed heat transfer measurements were performed by using 178 thermocouples in a channel with pin fin array. Local heat transfer coefficients and local heat transfer enhancement coefficients were obtained for eight Reynolds numbers ranging from 2000 to 100,000 on the endwall of the channel. The endwall boundary conditions for heat transfer investigation are heating the bottom endwall and heating symmetrically the bottom and top endwalls with constant heat flux. The mechanism of heat transfer enhancement with pin fin array has been discussed. © 2001 Scripta Technica, Heat Trans Asian Res, 30(7): 533–541, 2001     

圆柱阵列换热器底部壁面对流换热的初步研究

圆柱阵列换热器是最具潜力的太阳能电站换热器。该文分析了圆柱阵列换热器底部壁面对流换热特性。编制了基于最小二乘法的计算程序，并据此利用现有实验数据拟合出具有普遍意义的圆柱阵列换热器底部壁面对流换热的无量纲换热参数的表达式，对比分析表明，该表达式真实地反映了圆柱阵列的存在及其尺度的变化对底部壁面换热的影响。这一结果为优化换热器结构和建立研究太阳能换热器的多区域连续数值模型奠定了基础。     

The Flow Resistance and Heat Transfer Characteristics of Micro Pin-Fins with Different Cross-Sectional Shapes

The flow resistance and heat transfer characteristics of deionized water flowing through a rectangular channel (60 mm × 5.2 mm × 0.5 mm) with staggered array micro pin-fin circular, diamond, and elliptical groups are experimentally investigated over Reynolds numbers ranging from 8 to 1,000, and the investigation shows that the flow resistance increases due to the endwall effect and large pin-fins density at low Re. With the increase in Re, the endwall effect is weakened, but the flow resistance still increases due to the appearance of vortex resistance, and the heat transfer is enhanced due to the flow disturbance or transition from laminar flow to turbulent flow. The experimental results are also compared with predictions of the theoretical correlations for the staggered array micro pin-fin groups, and the comparisons indicate that only the correlation related to the diamond shaped micro pin-fin groups approximately agrees with experimental data, and the other correlations do not describe well the flow and heat transfer characteristics covering laminar, transitional, and turbulent states in circular and elliptical test sections.     

Natural convection heat transfer from a heat sink with hollow/perforated circular pin fins

Experiments were performed on natural convection heat transfer from circular pin fin heat sinks subject to the influence of its geometry, heat flux and orientation. The geometric dependence of heat dissipation from heat sinks of widely spaced solid and hollow/perforated circular pin fins with staggered combination, fitted into a heated base of fixed area is discussed. Over the tested range of Rayleigh number, 3.8 × 10⁶ ≤ Ra ≤ 1.65 × 10⁷, it was found that the solid pin fin heat sink performance for upward and sideward orientations shows a competitive nature, depending on Rayleigh number and generally shows higher heat transfer coefficients than those of the perforated/hollow pin fin ones in both arrangement. For all tested hollow/perforated pin fin heat sinks, however, the performance for sideward facing orientation was better than that for upward facing orientation. This argument is supported by observing that the augmentation factor was around 1.05–1.11, depending on the hollow pin diameter ratio, D_i/D_o. Meanwhile, the heat sink of larger hollow pin diameter ratio, D_i/D_o offered higher heat transfer coefficient than that of smaller D_i/D_o for upward orientation, and the situation was reversed for sideward orientation. The heat transfer performance for heat sinks with hollow/perforated pin fins was better than that of solid pins. The temperature difference between the base plate and surrounding air of these heat sinks was less than that of solid pin one and improved with increasing D_i/D_o.     

Experimental Verification of Model for Liquid-Cooled Staggered Pin Fin Heat Sinks with Top Bypass Flow

Pressure drops and heat transfer over staggered pin fin heat sinks with top bypass flow were experimentally evaluated. The authors considered liquid-cooling applications because there were few data available comparing to air-cooling applications. Empirical equations to predict heat transfer on the endwall were developed by obtaining experimental data on the copper base plate with acrylic pins. A new model for predicting pressure drops and heat transfer over staggered pin fin heat sinks with top bypass flow based on mass, momentum, and energy conservation within the two control volumes is proposed. The first control volume in the model is located within the finned area, and the second is located in the gap between the tip of the pins and the flow channel. This model combines two conditions according to the boundary-layer thickness. A comparison between experimental and calculated results revealed that dimensionless pressure drops and the Nusselt number could be predicted within 30% error for the former and 50% error for the latter.     

Secondary flow and forced convection heat transfer near endwall boundary layer fences in a 90° turning duct

The present study deals with the secondary flow and heat transfer aspects of endwall boundary layer fences in 90° turning ducts. Boundary layer fences have recently been re-introduced as additional components to achieve favorable aerothermal effects in turbine/compressor passages and internal cooling systems. The turbine passage or internal coolant channel is simulated by a 90° turning duct (Re_D=360,000) to study the aerothermal interaction of the boundary layer fence and the passage flow that is dominated by the passage vortices. Specifically, a single boundary layer fence of varying dimensions is attached to a heated endwall of the duct. The current study adds to the currently small number of investigations on the use of endwall boundary layer fences and their aerothermal interaction with the passage flow system.     

Three Dimensional Numerical Investigation on the Effect of Wedge Angle of a Passage with Pin-fin Arrays

Heat transfer in passage with pin-fin arrays for cooling blade trailing edge was studied numerically. Three-dimensional numerical simulations were carried out for steady laminar flow in passages with different wedge angles between pressure surface and suction surface of cooling blade trailing edge to study the effect of different wedge angles (from 0°to 30°) on heat transfer and pressure losses. Research was carried out for both in-line array and staggered array. From this investigation, wedge angle 10°gives the best heat transfer performance.     

Flow structure and heat transfer characteristics of a 90°-turned pin-finned wedge duct with dimples at different locations

In this study, numerical simulations are performed to investigate the effect of dimple location on the flow structure and heat transfer characteristics in a 90°-turned channel with pin fins. Results of the flow structure, heat transfer on the endwall, and friction factor are included in this study. It is found that the dimple location has an impact on the flow structure and heat transfer both for the pin fins arranged with in-line and staggered layout. The horseshoe vortex near the pin fin is influenced by the location of the dimple. The dimple enhances the Nusselt number on the endwall surface significantly. In addition, the dimple location has an effect on the low-speed recirculation, impingement, and vortex which are generated by the dimple. However, the dimple has very limited effects on the friction factor for all cases. It is also found that the area goodness factor and volume goodness factor are improved by the dimple.     

Heat transfer correlations of perpendicularly impinging jets on a hemispherical-dimpled surface

Heat transfer results of an inline array of round jets impinging on a staggered array of hemispherical dimples are reported with the consideration of various parametric effects such as Reynolds number (Re_Dj), jet-to-plate spacing (H/D_j), dimple depth (d/D_d) and ratio of jet diameter to dimple projected diameter (D_j/D_d) for both impinging on dimples and impinging on flat portions. The results were normalized against those from a flat plate. The heat transfer was measured by using transient wideband liquid crystal method. Our previous work (Kanokjaruvijit and Martinez-Botas (2005) [1]) on the effect of crossflow scheme suggested that jet impingement coupled with channel-like flow formed by the crossflow helped enhance heat transfer on a dimpled surface; hence three sidewalls were installed to constrain the spent air to leave in one direction. Throughout the study, the pitch of the nozzle holes was kept constant at 4 jet diameters. The Reynolds number (Re_Dj) ranging from 5000 to 11,500, jet-to-plate spacing (H/D_j) varying from 1 to 12 jet diameters, dimple depths (d/D_d) of 0.15, 0.25 and 0.29, and dimple curvature (D_j/D_d) of 0.25, 0.50 and 1.15 were examined. The shallow dimples (d/D_d = 0.15) improved heat transfer significantly by 70% at H/D_j = 2 compared to that of the flat surface, while this value was 30% for the deep ones (d/D_d = 0.25). The improvement also occurred to the moderate and high D_j/D_d. Thereafter, the heat transfer results were correlated in dimensionless form by using logarithmic multiple regression. The correlations were reported with necessary statistics.     

An experimental study of pressure loss and heat transfer in the pin fin-dimple channels with various dimple depths

An experimental study was conducted to investigate the effects of dimple depth on the pressure loss and heat transfer characteristics in a pin fin-dimple channel, where dimples are located on the endwall transversely between the pin fins. The pin fin-dimple channels considered consist of ten rows of pin fin-dimple combined structure. The pin fin transverse spacing-to-diameter ratio S/D = 2.5, the streamwise spacing-to-diameter ratio X/D = 2.5, the pin fin height-to-diameter ratio H/D = 1.0. The dimples have a print diameter the same with the pin fins, but have three different dimple depth-to-diameter ratios, i.e. δ/D = 0.1, 0.2 and 0.3. The experimental results, mainly the average Nusselt number and friction factor, for the pin fin-dimple channels with various dimple depths have been obtained and compared with each other for the Reynolds number range of 8200–50,500. The study showed that, compared to the baseline pin fin channel, the pin fin-dimple channels have further improved convective heat transfer performance by up to 19.0%, and the pin fin-dimple channel with deeper dimples shows relatively higher Nusselt number values. The study still showed dimple depth-dependent pressure loss behaviors for the pin fin-dimple channels compared to the pin fin channel, and the pin fin-dimple channel with shallower dimples shows relatively lower friction factors by up to 17.6% over the studied Reynolds number range. Furthermore, three-dimensional conjugate computations have been carried out for similar experimental conditions, and the computations showed the detailed characteristics in the distribution of the velocity and turbulence level in the flow, which revealed the underlying mechanisms for the associated dimple depth-dependent pressure loss and heat transfer characteristics in the pin fin-dimple channels.     

Heat transfer characteristics of an array of impinging pre-mixed slot flame jets

An experimental study was carried out to investigate the shape and the heat transfer characteristics of an array of three laminar pre-mixed butane/air slot flame jets impinging upwards normally on a horizontal water-cooled flat plate. The effects of jet-to-jet spacing and nozzle-to-plate distance were examined at the Reynolds number (Re) of 1000 and the equivalence ratio (?) of unity. Comparisons of the heat transfer characteristics between single and multiple slot flame jets, as well as multiple slot and round jets, were made. The between-jet interference decreased with increasing jet-to-jet spacing (s/d_e) and nozzle-to-plate distance (H/d_e). Strong interference was obtained at s/d_e = 1 and H/d_e = 2, at which the central jet was suppressed while the side jets were deflected towards their free sides. In addition, there was no minimum heat flux found in the inter-jet interacting zone, instead, a peak heat flux was obtained. Thermal performance was reduced when H/d_e became smaller than the length of the conical luminous reaction zone of the flame. A maximum average heat flux occurred at the moderate jet-to-jet spacing of s/d_e = 2.5 at Re = 1000, ? = 1 and H/d_e = 2. The resultant heat flux distribution of the central jet of a multiple slot jets system was higher than that of a single slot jet when the jet-to-jet spacing was small, but this advantage in thermal performance diminished when the jet-to-jet spacing was increased. Besides, the area-averaged heat flux of the multiple slot flame jets was higher than that of the multiple round flame jets arranged at the same geometric configuration.     

Thermal performance assessment of turbulent channel flows over different shaped ribs

Experiments are conducted to assess turbulent forced convection heat transfer and friction loss behaviors for air flow through a constant heat flux channel fitted with different shaped ribs. The rib cross-sections used in the present study are triangular (isosceles), wedge (right-triangular) and rectangular shapes. Two rib arrangements, namely, in-line and staggered arrays, are introduced. Measurements are carried out for a rectangular channel of aspect ratio, AR = 15 and height, H = 20 mm with single rib height, e = 6 mm and rib pitch, P = 40 mm. The flow rate is in terms of Reynolds numbers based on the inlet hydraulic diameter of the channel in a range of 4000 to 16,000. The experimental results show a significant effect of the presence of the ribs on the heat transfer rate and friction loss over the smooth wall channel. The in-line rib arrangement provides higher heat transfer and friction loss than the staggered one for a similar mass flow rate. In comparison, the wedge rib pointing downstream yields the highest increase in both the Nusselt number and the friction factor but the triangular rib with staggered array shows better thermal performance over the others.     

Combustion characteristics and thermal enhancement of premixed hydrogen/air in micro combustor with pin fin arrays

Targeted at improving the combustion stability and enhancing heat transfer in micro combustor, the combustion characteristics and thermal performance of micro combustor with pin fin arrays are numerically investigated by employing detail H₂/O₂ reaction mechanism. It is shown that the micro combustor with staggered pin fin arrays exhibits the highest average temperature and heat flux of external wall, while the micro combustor with in-line pin fin arrays displays the most uniform temperature distribution of external wall. When the equivalence ratio is 1.1, all micro combustors exhibit the highest mean temperature and heat flux of external wall. The micro combustor materials with high thermal conductivity can not only improve the average temperature and heat flux of external wall, but also enhance heat transfer to the upstream which can preheat the mixed gas. Therefore, the materials with high thermal conductivity, such as red copper and aluminum, can make up for the nonuniform temperature distribution of micro combustor with staggered pin fin arrays, so as to realize uniform high heat flux output of external wall.     

Convective heat transfer and pressure losses across novel heat sinks fabricated by Selective Laser Melting

This study presents the thermal and fluid flow characteristics of five heat sinks that have been fabricated by a rapid manufacturing technique known as Selective Laser Melting. The five heat sinks consist of two conventional designs, the cylindrical pin and rectangular fin array, for comparison purposes, and three novel heat sinks: a staggered elliptical array; a lattice; and a rectangular fin array with rounded corners. The experimental results for the rectangular fin were compared with data from the literature and were found to be consistent. The rectangular fin with rounded corners proved able to transfer the largest amount of heat whilst improving upon the pressure drop performance of the standard rectangular fin array. Although the lattice arrangement made use of the fabrication process’ ability to manufacture heat sinks with high surface area to volume ratios, its performance was limited by the lack of interaction between the cooling air and structure. In terms of both heat transfer performance and pressure drop, the staggered elliptical array, which cannot be manufactured by conventional techniques, outperformed the other heat sinks.     

Heat transfer and friction factor correlation for artificially roughened duct with metal grit ribs

This paper presents the results of an experimental investigation of heat transfer to the airflow in the rectangular duct of an aspect ratio 10:1. The top wall surface is made rough with metal ribs of circular cross section in staggered manner to form defined grid. The roughened wall is uniformly heated and the other walls are insulated. This geometry of duct closely corresponds to that used in solar air heaters. The effect of grit geometry [i.e., relative roughness height of grid (e/D_h), relative roughness pitch of grit (p/e), relative length of grit (l/s)] on the heat transfer coefficient and friction factor is investigated. The range of variation of system parameters and operating parameters is investigated within the limits, as e/D_h: 0.035 to 0.044, p/e: 12.5–36 and l/s: 1.72–1, against variation of Reynolds number: 4000–17,000. It is observed that the plate of roughness parameters l/s = 1.72, e/D_h = 0.044, p/e = 17.5 shows optimum performance. Correlations for Nusselt number and friction factor in terms of above parameters are developed which reasonably correlate the experimental data.     

有扰流片的矩形通道内空气流动和传热过程的数值模拟

以高温透叶片尾部区内部冷却为应用背景，对带顺排、错排扰流片肋的通道内空气流动和传热过程进行了数值模拟。计算结果表明，在相同雷诺数下，错排扰流片的阻力系数比针肋和顺排绕流片的阻力因子均增大约2%，而冷却能力分别增大约50%和9%。