Heat Transfer Rate Performance Research Articles

Effect of Prandtl Number on Free Convection in a Solar Collector Filled with Nanofluid

Numerical study of the influence of Prandtl number on free convection flow phenomena in a solar collector having glass cover plate and sinusoidal absorber is done. The working fluid is water-Al2O3 nanofluid. The cover plate has initially constant temperature Th, while bottom absorber is at temperature Tc, with Th > Tc. The remaining walls are considered adiabatic. By Penalty Finite Element Method the governing differential equations with boundary conditions are solved. The effect of the Prandtl number on the flow pattern and heat transfer has been depicted. Comprehensive average Nusselt number, average temperature and mean velocity inside the collector are presented as a function of the governing parameter mentioned above. The highest Pr causes the greatest heat transfer. The enhancing performance of heat transfer rate is more effective for the water-Al2O3 nanofluid than the base fluid.

Flow and heat transfer in serpentine channels

Serpentine channels are often used in microchannel reactors and heat exchangers. These channels offer better mixing, higher heat and mass‐transfer coefficients than straight channels. In the present work, flow and heat transfer experiments were carried out with a serpentine channel plate comprising of 10 units (single unit dimensions: 1 × 1.5 mm2 in cross section, length 46.28 mm, Dh 1.2 mm) in series. Pressure drop and heat‐transfer coefficients were experimentally measured. Flow and heat transfer in the experimental set‐up were simulated using computational fluid dynamics (CFD) models to understand the mechanisms responsible for performance enhancement. The CFD methodology, thus, developed was applied to understand the effect of various geometrical parameters on heat transfer enhancement. A criterion was defined for evaluation of heat transfer performance (heat transfer per unit pumping power), thus, ensuring due considerations to required pumping power. The effect of geometrical parameters and the corresponding mechanisms contributing for enhancement are discussed briefly. Based on the results, a design map comprising different serpentine channels showing heat transfer enhancement with pumping power was developed for Reynolds number of 200 which will be useful for further work on flow and heat transfer in serpentine channels. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1814–1827, 2013

The approach of minimum heat consumption and its applications in convective heat transfer optimization

Based on the analysis of the irreversibility of heat transfer process, a physical quantity called enerty is introduced to establish an equilibrium equation which is different from conventional energy conservation equation. Comparing to minimum entropy generation and minimum entransy dissipation, an approach of minimum heat consumption is proposed to optimize heat transfer process. The transport efficiency, a criterion to evaluate heat transfer performance, is proposed based on the definition of heat consumption rate. By setting minimum heat consumption as optimization objective and fluid power consumption as constraint condition, a momentum equation with additional volume force is constructed through functional variation to numerically simulate convective heat transfer in coupling with energy equation. The results disclose that longitudinal swirl flow with single vortex or multi-vortexes appears in the flow field, which leads to heat transfer enhancement. Additionally, a sub-area method is developed to enhance convective heat transfer characterized by relatively higher Nu number, which can be applied to optimize flow field in a circular tube and guide the design for tube insert. The results further indicate that the area ratio and the intensity of additional volume force affect the distribution of temperature and velocity fields. A comparison among minimum heat consumption (MHC), minimum entransy dissipation (MED) and minimum power consumption (MPC) is conducted. The results show that heat transfer performances are close to each other for using the three approaches, which indicates that the proposed MHC approach is effective in designing tube insert or heat transfer unit.

Heat transfer performance evaluation for turbulent flow through a tube with twisted wire brush inserts

In the present study, the heat transfer performance and friction factor characteristics in a circular tube fitted with twisted wire brush inserts were investigated experimentally. The twisted wire brush inserts were fabricated with four different twisted wire densities of 100, 150, 200, and 250 wires per centimeter by winding a 1mm diameter of the copper wire over a 5mm diameter of two twisted iron core-rods. Heat transfer and friction factor data in tubes were examined for Reynolds number ranging from 7,200 to 50,200. The results indicated that the presence of twisted wire brush inserts led to a large effect on the enhancement of heat transfer with corresponding increase in friction factor over the plain tube. The Nusselt number and friction factor of using the twisted wire brush inserts were found to be increased up to 2.15 and 2.0 times, respectively, than those over the plain tube values. Furthermore, the heat transfer performance was evaluated to assess the real benefits of using those type of inserts and the performance was achieved 1.85 times higher compared to the plain tube based on the constant blower power. Finally, correlations were developed based on the data generated from this work to predict the heat transfer, friction factor, and thermal performance factor for turbulent flow through a circular tube fitted with the twisted wire brush inserts in terms of wire density (y), Reynolds number (Re), and Prandtl number (Pr).

0414 マイクロチャンネルを用いた次世代熱交換器の開発(S46-1 高効率ヒートポンプの要素技術とその応用(1),S46 高効率ヒートポンプの要素技術とその応用)

It is requested to develop a small and highly efficient heat exchanger for small size energy equipment such as fuel cells and CO_2 heat pumps. In the present study, the high pressure resistant microchannel stacked heat exchanger had been developed. The present device manufactured by diffused bond method can withstand high pressure at least 15MPa The heat transfer area per unit volume of the device is quite large. Since the distance between the microchannels in the device is small, the thermal resistance is small. Due to the high pressure resistance, this device can be applied for high flow rate condition with boiling and condensation. As the results, it is clarified that above features make the device to achieve very high performance of heat transfer rate with hundreds times larger than that of the existing heat exchanger.

606 並列ループ型熱サイフォンの伝熱性能に関する研究(関東支部 茨城講演会)

A parallel looped thermosyphon is newly developed to improve heat transfer defects of a conventional thermosyphon. The purpose of this study is to examine experimentally and to evaluate heat transfer performance of the new thermosyphon. Experiments are conducted with water working fluid at charge ratios of 0.5 and 0.9 and coolant temperatures of 25 to 50℃. Major findings are follows: The higher coolant temperature produces the higher critical heat flux. The heat transfer coefficient at 0.9 of the charge ratio higher than that at 0.5.

Experimental investigation on the performance of air cooler under frosting conditions

Evaluation of heat transfer performance of the air cooler under frosting conditions is of great importance for the refrigeration industry. In this paper, effects of frost growth on the performance of the air cooler have been studied with an experimental air cooler of industrial size with different fin spacings, i.e., 6, 8 and 10 mm. Results showed that factors affecting the heat transfer performance of the air cooler include the evaporation temperature, the frost height, the fin spacing and the air velocity. The overall heat transfer coefficients based on a logarithmic mean temperature difference (LMTD) and the energy transfer coefficients based on a logarithmic mean enthalpy difference (LMED) were calculated. As the frost accumulated on the air cooler, the overall heat transfer coefficient and energy transfer coefficient will drop gradually.

A117 正弦波状壁面を有する流路内流れの熱流動特性に関する数値解析的研究(オーガナイズドセッション2 : 複雑な流れにおける熱・物質伝達)

Numerical simulation on the flow and heat transfer of sinusoidal wavy channel was conducted to investigate the effect of phase-difference between upper and lower wall. The difference was varied from 0 to π. The phase difference induces the dynamic change of flow and heat transfer characteristics. The critical Reynolds number at which flow changes its state from steady to unsteady was found to be lower for a larger phase difference. The heat transfer performance evaluation at constant pumping power base showed that the largest phase difference case(π) was the most preferable among five cases studied.

223 自動車用ヘッドランプの熱流体解析

The heat transfer performance evaluation and flow field prediction in an automobile headlamp are very important from the view points of the heatproof ability, durability and ventilation performance improvements of the headlamp. In the present research, the heat transfer performance and flow field in an automobile headlamp model was investigated both experimentally and numerically. The Particle Image Velocimetry (PIV) was used to measure the velocity field of the heat transfer loop in the headlamp at first. Then, CFD simulation was conducted by using the PIV measurement results as the boundary condition of the CFD simulation.

An Experimental Study on the Performance of a Perforated-Surface Heat Exchanger with Passage Enlargement and Contraction (An Evaluation of Heat Transfer Performance)

An Experimental Study on the Performance of a Perforated-Surface Heat Exchanger with Passage Enlargement and Contraction (An Evaluation of Heat Transfer Performance)

An experimental study on the performance of a perforated duct with enlargement and contraction. An evaluation of heat transfer performance.

A new concept on the enhancement of forced-convection heat transfer at a Reynolds number lower than 3000 is presented. A new surface has many holes and is bent in a trapezoid shape. The phase of each adjacent surface is changed by half a period, and therefore a duct constructed with these surfaces has enlargement and contraction parts alternately along the flow passages. The heat transfer coefficient of the new duct increased by about three times that of a duct with plain surfaces at the same Reynolds number.