Effect of Pin Density on Heat-Mass Transfer and Fluid Flow at Low Reynolds Numbers in Minichannels

2010 ◽  
Vol 132 (6) ◽  
Author(s):  
N. K. C. Selvarasu ◽  
Danesh K. Tafti ◽  
Neal E. Blackwell
Keyword(s):  
Heat Transfer ◽  
Heat Capacity ◽  
Entropy Generation ◽  
Plane Channel ◽  
Performance Measure ◽  
Tip Clearance ◽  
Turbulent Regime ◽  
Pin Fin ◽  
Pin Fins ◽  
Optimal Spacing

Previous investigations on the performance of straight pins, pins with tip clearance, and profiled fins showed that closely packed cylindrical pin fins are very competitive with the modified pins. Therefore, the objective of this paper is to investigate the effect of pin density on performance. Steady/time-dependent calculations are performed to investigate the effect of pin density on friction and heat transfer. Pins packed at distances of SD=1.1, 1.2, 1.3, 1.4, 1.5, 2.0, 2.5, and 3 pin diameters (D) are investigated for 10≤ReD≤600. Two performance measures are used to compare the different pin fin densities. The first measure is to maximize heat transfer capacity for a given pumping power compared with a plane channel. The second measure used is based on entropy generation minimization (EGM), where the objective is to reduce the total irreversibility of the pin fin array to obtain an optimal spacing. Based on the performance measure of maximizing heat capacity, it is shown that for plain channels operating in the laminar range using denser pin packing has distinct advantages with SD=1.1 providing the best augmentation. However, the augmentation in heat capacity becomes relatively independent of the pin density for a channel operating in the turbulent regime. Based on the EGM method, at ReD>200, SD=1.3, 1.4, and 1.5 are the most suitable, with the least entropy generation observed at SD=1.4. At ReD<200, SD=1.1, 1.2, and 1.3 are also suitable for keeping entropy generation low.

Experimental Measurements of the Flow and Heat Transfer of a Square Jet Impinging on an Array of Square Pin Fins

2005 ◽  
Vol 128 (1) ◽  
pp. 61-70 ◽  
Author(s):  
Johnny S. Issa ◽  
Alfonso Ortega
Keyword(s):  
Heat Transfer ◽  
Flow Behavior ◽  
Jet Impingement ◽  
Tip Clearance ◽  
Heat Sinks ◽  
Clearance Ratio ◽  
Pressure Loss Coefficient ◽  
Pin Fin ◽  
Air Jet ◽  
Pin Fins

The flow behavior and heat transfer due to free air jet impingement on pin fin heat sinks was experimentally studied. Flow velocities and tip clearance ratios were varied from 2to20m∕s and 0 to 1, respectively. The stagnation pressure recovered at the center of the heat sink was higher for tall pins than for short pins. The pressure loss coefficient showed little dependence on Re, increased with increasing pin density and pin diameter, and decreased with increasing pin height and clearance ratio. The overall base-to-ambient thermal resistance decreased with increasing Re number, pin density, and pin diameter.

Optimization of Pin-Fins for a Heat Exchanger by Entropy Generation Minimization and Constructal Law

2015 ◽  
Vol 137 (6) ◽  
Author(s):  
Gongnan Xie ◽  
Yidan Song ◽  
Masoud Asadi ◽  
Giulio Lorenzini
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Entropy Generation ◽  
Thermal Energy ◽  
Pin Fin ◽  
Entropy Generation Minimization ◽  
Flow And Heat Transfer ◽  
Constructal Law ◽  
Pin Fins ◽  
Computational Fluid Dynamics Cfd

Pin-fins are considered as one of the best elements for heat transfer enhancement in heat exchangers. In this study, the topology of pin-fins (length, diameter, and shape) is optimized based on the entropy generation minimization (EGM) theory coupled with the constructal law (CL). Such pin-fins are employed in a heat exchanger in a sensible thermal energy storage (TES) system so as to enhance the rate of heat transfer. First, the EGM method is used to obtain the optimal length of pin-fins, and then the CL is applied to get the optimal diameter and shape of pin-fins. Reliable computational fluid dynamics (CFD) simulations of various constructal pin-fin models are performed, and detailed flow and heat transfer characteristics are presented. The results show that by using the proposed system with optimized pin-fin heat exchanger the stored thermal energy can be increased by 10.2%.

Effect of Pin Tip Clearance on Flow and Heat Transfer at Low Reynolds Numbers

2008 ◽  
Vol 130 (7) ◽  
Author(s):  
Ali Rozati ◽  
Danesh K. Tafti ◽  
Neal E. Blackwell
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Tip Clearance ◽  
Number Range ◽  
Pin Fin ◽  
Flow And Heat Transfer ◽  
Pin Fins ◽  
Low Reynolds ◽  
Flow Effects ◽  
High Reynolds

Cylindrical pin fins with tip clearances are investigated in the low Reynolds number range 5<ReD<400 in a plane minichannel. Five tip gaps are investigated ranging from a full pin fin (t*=0.0) to a clearance of t*=0.4D*, where D* is the pin diameter. It is established that unlike high Reynolds number flows, the flow and heat transfer are quite sensitive to tip clearance. A number of unique flow effects, which increase the heat transfer performance, are identified. The tip gap affects the heat transfer coefficient by eliminating viscosity dominated end wall effects on the pin, by eliminating the pin wake shadow on the end walls, by inducing accelerated flow in the clearance, by reducing or impeding the development of recirculating wakes, and by redistributing the flow along the height of the channel. In addition, tip gaps also reduce form losses and friction factor. A clearance of t*=0.3D* was found to provide the best performance at ReD<100; however, for ReD>100, both t*=0.2D* and 0.3D* were comparable in performance.

scholarly journals Heat Transfer From Low Aspect Ratio Pin Fins

2007 ◽  
Author(s):  
Michael E. Lyall ◽  
Alan A. Thrift ◽  
Atul Kohli ◽  
Karen A. Thole
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Gas Turbines ◽  
Channel Height ◽  
Internal Cooling ◽  
Number Range ◽  
Pin Fin ◽  
Heat Transfer Augmentation ◽  
Low Aspect Ratio ◽  
Pin Fins

The performance of many engineering devices from power electronics to gas turbines is limited by thermal management. Heat transfer augmentation in internal flows is commonly achieved through the use of pin fins, which increase both surface area and turbulence. The present research is focused on internal cooling of turbine airfoils using a single row of circular pin fins that is oriented perpendicular to the flow. Low aspect ratio pin fins were studied whereby the channel height to pin diameter was unity. A number of spanwise spacings were investigated for a Reynolds number range between 5000 to 30,000. Both pressure drop and spatially-resolved heat transfer measurements were taken. The heat transfer measurements were made on the endwall of the pin fin array using infrared thermography and on the pin surface using discrete thermocouples. The results show that the heat transfer augmentation relative to open channel flow is the highest for smallest spanwise spacings and lowest Reynolds numbers. The results also indicate that the pin fin heat transfer is higher than the endwall heat transfer.

Heat Transfer and Pressure Loss Characteristics of Pin-Fins With Different Shapes in a Wide Channel

2017 ◽  
Author(s):  
Jin Xu ◽  
Jiaxu Yao ◽  
Pengfei Su ◽  
Jiang Lei ◽  
Junmei Wu ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Thermal Performance ◽  
Pressure Loss ◽  
Bottom Surface ◽  
Number Range ◽  
Pin Fin ◽  
Nominal Diameter ◽  
Pin Fins

Convective heat transfer enhancement and pressure loss characteristics in a wide rectangular channel (AR = 4) with staggered pin fin arrays are investigated experimentally. Six sets of pin fins with the same nominal diameter (Dn = 8mm) are tested, including: Circular, Elliptic, Oblong, Dropform, NACA and Lancet. The relative spanwise pitch (S/Dn = 2) and streamwise pitch (X/Dn = 4.5) are kept the same for all six sets. Same nominal diameter and arrangement guarantee the same blockage area in the channel for each set. Reynolds number based on channel hydraulic diameter is from 10000 to 70000 with an increment of 10000. Using thermochromic liquid crystal (R40C20W), heat transfer coefficients on bottom surface of the channel are achieved. The obtained friction factor, Nusselt number and overall thermal performance are compared with the previously published data from other groups. The averaged Nusselt number of Circular pin fins is the largest in these six pin fins under different Re. Though Elliptic has a moderate level of Nusselt number, its pressure loss is next to the lowest. Elliptic pin fins have pretty good overall thermal performance in the tested Reynolds number range. When Re>40000, Lancet has a same level of performance as Circular, but its pressure loss is much lower than Circular. These two types are both promising alternative configuration to Circular pin fin used in gas turbine blade.

Heat Transfer Characteristics of a Flow Passage With Long Pin Fins and Improving Heat Transfer Coefficient by Adding Turbulence Promoters on a Endwall

2001 ◽  
Author(s):  
K. Takeishi ◽  
T. Nakae ◽  
K. Watanabe ◽  
M. Hirayama
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Aspect Ratio ◽  
Transfer Coefficient ◽  
High Heat ◽  
Atmospheric Conditions ◽  
Pin Fin ◽  
High Heat Transfer ◽  
Turbine Vanes ◽  
Pin Fins

Pin fins are normally used for cooling the trailing edge region of a turbine, where their aspect ratio (height H/diameter D) is characteristically low. In small turbine vanes and blades, however, pin fins may also be located in the middle region of the airfoil. In this case, the aspect ratio can be quite large, usually obtaining values greater than 4. Heat transfer tests, which are conducted under atmospheric conditions for the cooling design of turbine vanes and blades, may overestimate the heat transfer coefficient of the pin-finned flow channel for such long pin fins. The fin efficiency of a long pin fin is almost unity in a low heat transfer situation as it would be encountered under atmospheric conditions, but can be considerably lower under high heat transfer conditions and for pin fins made of low thermal conductivity material. A series of tests with corresponding heat transfer models has been conducted in order to clarify the heat transfer characteristics of the long pin-finned flow channel. It is assumed that heat transfer coefficients can be predicted by the linear combination of two heat transfer equations, which were separately developed for the pin fin surface and for tubes in crossflow. To confirm the suggested combined equations, experiments have been carried out, in which the aspect ratio and the thermal conductivity of the pin were the test parameters. To maintain a high heat transfer coefficient for a long pin fin under high-pressure conditions, the heat transfer was augmented by adding a turbulence promoter on the pin-finned endwall surface. A corresponding equation that describes this situation has been developed. The predicted and measured values showed good agreement. In this paper, a comprehensive study on the heat transfer of a long pin-fin array will be presented.

Effect of Absorptive Coating of the Hot Fluid Passage of a Heat Exchanger Employing Exergy-Optimized Pin Fins in High Temperature Applications

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
Nwosu P. Nwachukwu ◽  
Samuel O. Onyegegbu
Keyword(s):  
Heat Transfer ◽  
High Temperature ◽  
Simple Relation ◽  
Base Temperature ◽  
Minimum Entropy ◽  
Pin Fin ◽  
Temperature Heat ◽  
Pin Fins ◽  
Minimum Entropy Generation ◽  
Fluid Parameters

An expression for the optimum pin fin dimension is derived on exergy basis for a high temperature exchanger employing pin fins. The present result differs from that obtained by Poulikakos and Bejan (1982, “Fin Geometry for Minimum Entropy Generation in Forced Convection,” ASME J. Heat Transfer, 104, pp. 616–623) for a low temperature heat recovery application. Also, a simple relation is established between the amounts the base temperature of the optimized pin fin is raised for a range of absorptive coating values. Employing this relation, if the absorptivity of the coating, the plate emissivity, the number of protruding fins, and some area and fluid parameters are known, the corresponding value for the base temperature of the fin is immediately obtained. The analysis shows that the thermal performance of the exchanger improves substantially with a high absorptivity coating hence can be seen as a heat transfer enhancement feature of the exchanger operating with radiation dominance.

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