scholarly journals Laminar convective heat transfer for in-plane spiral coils of noncircular cross sections ducts: A computational fluid dynamics study

2012 ◽  
Vol 16 (1) ◽  
pp. 109-118 ◽  
Author(s):  
Jundika Kurnia ◽  
Agus Sasmito ◽  
Arun Mujumdar
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Cross Sections ◽  
Wall Temperature ◽  
Heat Transfer Performance ◽  
Constant Heat Flux ◽  
Uniform Heat Flux ◽  
Transfer Performance ◽  
Constant Wall Temperature ◽  
Flux Boundary Condition

The objective of this study was to carry out a parametric study of laminar flow and heat transfer characteristics of coils made of tubes of several different cross-sections e.g. square, rectangular, half-circle, rectangular and trapezoidal. For the purpose of ease of comparison, numerical experiments were carried out base on a square-tube Reynolds number of 1000 and a fixed fluid flow rate while length of the tube used to make coils of different diameter and pitch was held constant. A figure of merit was defined to compare the heat transfer performance of different geometry coils; essentially it is defined as total heat transferred from the wall to the surroundings per unit pumping power required. Simulations were carried out for the case of constant wall temperature as well as constant heat flux. In order to allow reasonable comparison between the two different boundary conditions - constant wall temperature and constant wall heat flux - are tested; the uniform heat flux boundary condition was computed by averaging the heat transferred per unit area of the tube for the corresponding constant wall temperature case. Results are presented and discussed in the light of the geometric effects which have a significant effect on heat transfer performance of coils.

Laminar Slug Flow: Heat Transfer Characteristics With Constant Heat Flux Boundary

2009 ◽  
Author(s):  
P. A. Walsh ◽  
E. J. Walsh ◽  
Y. S. Muzychka
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Nusselt Number ◽  
Heat Transfer Performance ◽  
Constant Heat Flux ◽  
Transfer Performance ◽  
Segmented Flow ◽  
Constant Heat ◽  
Flux Boundary Condition ◽  
Flow Heat

The problem of elevated heat flux in modern electronics has led to the development of numerous liquid cooling devices which yield superior heat transfer coefficients over their air based counterparts. This study investigates the use of liquid/gas slug flows where a liquid coolant is segregated into discrete slugs, resulting in a segmented flow, and heat transfer rates are enhanced by an internal circulation within slugs. This circulation directs cooler fluid from the center of the slug towards the heated surface and elevates the temperature difference at the wall. An experimental facility is built to examine this problem in circular tube flow with a constant wall heat flux boundary condition. This was attained by Joule heating a thin walled stainless steel tube. Water was used as the coolant and air as the segregating phase. The flow rates of each were controlled using high precision syringe pumps and a slug producing mechanism was introduced for segmenting the flow into slugs of various lengths at any particular flow rate. Tube flows with Reynolds numbers in the range 10 to 1500 were examined ensuring a well ordered segmented flow throughout. Heat transfer performance was calculated by measuring the exterior temperature of the thin tube wall at various locations using an Infrared camera. Nusselt number results are presented for inverse Graetz numbers over four decades, which spans both the thermally developing and developed regions. The results show that Nu in the early thermally developing region are slightly inferior to single phase flows for heat transfer performance but become far superior at higher values of inverse Gr. Additionally, the slug length plays an important role in maximizing Nusselt number in the fully developed region as Nu plateaus at different levels for slugs of differing lengths. Overall, this paper provides a new body of experimental findings relating to segmented flow heat transfer in constant heat flux tubes without boiling. Put abstract text here.

Exergy Transfer Criteria on Enhanced Heat Transfer Performance Evaluation

2006 ◽  
Author(s):  
Shuang-Ying Wu ◽  
Yan Chen ◽  
You-Rong Li ◽  
Wen-Zhi Cui ◽  
Liao Quan
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Performance Evaluation ◽  
Wall Temperature ◽  
Evaluation Criteria ◽  
Constant Heat Flux ◽  
Transfer Performance ◽  
Enhanced Heat Transfer ◽  
Constant Wall Temperature ◽  
Exergy Transfer

Based on the exergy transfer performance analysis of forced convective heat transfer through a tube with constant heat flux/ constant wall temperature for thermally and hydrodynamic fully developed turbulent flow, extended performance evaluation criteria for enhanced heat transfer surfaces based on the exergy transfer theorem have been developed. An exergy transfer performance evaluation criterion ΔNue or ΔE, which is defined as the difference of exergy-transfer Nusselt number or exergy transfer rate before and after enhanced heat transfer, is put forward. By reference to spirally grooved tube, the effect of Reynolds number, structure parameters of tube, dimensionless wall temperature difference and heat flux on the exergy transfer process is discussed. The results show that the exergy transfer performance of enhanced heat transfer with constant wall temperature is quite different from that with constant wall heat flux. An effective approach for exergy transfer performance evaluation and the optimal process parameters and configuration choice of enhanced heat transfer tube are provided.

Testing of Rectangular Channels With Perturbed Entrances for Temperature Difference Under Constant Heat Flux

2011 ◽  
Author(s):  
Jacek Marek Matyja ◽  
Tunde Bello-Ochende
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Performance ◽  
Cold Water ◽  
Constant Heat Flux ◽  
Entrance Region ◽  
Average Error ◽  
Transfer Performance ◽  
Rectangular Duct ◽  
Constant Heat

In this paper convective heat transfer performance of various duct geometries are compared using theoretical and experimental analyses. The experiments stretch further by perturbing the entrance region of the 2:1 rectangular duct (both inwards and outwards) and to obtain the effect on heat transfer performance. The cross-sectional area and length of the ducts are fixed and constant heat flux is applied to the ducts while cold water is used as the flow stream. The laminar flow regime is analysed. The theoretical and experimental cases are in agreement, with slight deviances attributed to certain assumptions made during the theoretical analysis and non-ideal testing conditions. The analyses concludes that perturbing the entrance region of a standard rectangular duct, both inwards and outwards, has a visible increase in heat transfer performance. The inward perturbed duct shows the highest increase in performance. The average variation between the theoretical and experimental case is about 18% for constant heat flux. The average error imposed on the results due experimental equipment is about 3% for constant heat flux experiments.

Three Dimensional Numerical Simulation of Gas Heat Transfer in Rectangular Microchannels

2010 ◽  
Author(s):  
Xiaohong Yan ◽  
Qiuwang Wang
Keyword(s):  
Heat Transfer ◽  
Boundary Condition ◽  
Heat Flux ◽  
Aspect Ratio ◽  
Wall Temperature ◽  
Three Dimensional ◽  
Uniform Heat Flux ◽  
Flux Boundary Condition ◽  
Uniform Heat ◽  
Heat Flux Boundary Condition

Rectangular microchannel is the typical component of the micro heat exchangers and micro heat sinks. Three-dimensional compressible Navier-Stokes equations are solved for gas flow and heat transfer in microchannels under uniform heat flux boundary condition. The numerical methodology is based on the control volume SIMPLE scheme. It is found that the heat removal characteristic for compressible flow is better than the incompressible flow and it is not suitable to use conventionally defined Nu to measure the heat transfer characteristic for compressible heat transfer. The effect of the aspect ratio (width to height) on the cross-sectional averaged wall temperature and the Nu is negligible under the uniform heat flux boundary condition. However, the local uniformity of the wall temperature is significantly influenced by the aspect ratio. The square cross-section exhibits the best local uniformity of the wall temperature.

Heat Transfer Performance of LiF–NaF–KF Salt in a Corrugated Receiver Tube With Non-Uniform Solar Flux

2017 ◽  
Author(s):  
Chaxiu Guo ◽  
Dongwei Zhang ◽  
Junjie Zhou ◽  
Wujun Zhang ◽  
Xinli Wei
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Transfer Performance ◽  
Tube Wall ◽  
Uniform Heat Flux ◽  
Transfer Performance ◽  
Corrugated Tube ◽  
The Heat Transfer Coefficient

The heat flux on the receiver tube is non-uniform because of uneven solar flux and receiver structure, which causes overheating and thermal stress failure of receiver and affected safe operations of the Concentrated Solar Power (CSP) system. In order to reduce the temperature difference in receiver tube wall and improve the efficiency of CSP system, the ternary eutectic salt LiF-NaF-KF (46.5-11.5-42 wt.%, hereafter FLiNaK), which has a better high thermal stability than that of nitrate salts at operating temperature of 900 °C, is selected as HTF, and heat transfer performance of FLiNaK in a corrugated receive tube with non-uniform heat flux is simulated by CFD software in the present work. The numerical results reveal that the non-uniform heat flux has a great influence on the temperature distributions of the receive tube and FLiNaK salt. Compared with the result of bare tube, the corrugated tube can not only significantly reduce the temperature difference in tube wall and salt by improving the uniformity of temperature distribution but also enhance the heat transfer of the salt, where the heat transfer coefficient increases with the Reynolds number and heat flux. Moreover, the enhanced effect of the corrugated tube depends on both the pitch and the height of ridges. It is found that the heat transfer coefficient of the salt gets a maximum when the ratio of the height of ridge to the pitch is 0.2. The research presented here may provide guidelines for design optimization of receiver tube in CSP system.

scholarly journals Thermal-Fluid Transport Phenomena of a Strongly-Heated Gas Flow in Parallel Tube Rotation

1998 ◽  
Vol 4 (4) ◽  
pp. 271-282 ◽  
Author(s):  
Shuichi Torii ◽  
Wen-Jei Yang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Gas Flow ◽  
Heat Transfer Performance ◽  
Transport Phenomena ◽  
Temperature Fields ◽  
Uniform Heat Flux ◽  
High Heat Flux ◽  
Rotational Axis ◽  
Transfer Performance

A numerical analysis is performed to study thermal transport phenomena in gas flow through a strongly heated tube whose axis is in parallel with the rotational axis. The velocity and temperature fields prevail when fluid flows in a rotating tube with uniform heat flux on the tube wall. The two-equationk-ωturbulence andt2¯-εtheat transfer models are employed to determine turbulent viscosity and eddy diffusivity for heat, respectively. The governing boundary-layer equations are discritized by means of a control volume finitedifference techniques. It is found that the Coriolis and centrifugal (or centripetal) forces cause fluid flow and heat transfer performance in the parallel-rotation system to be drastically different from those in the stationary case. Consequently, even if a tube rotating around a parallel axis is heated with high heat flux whose level causes a laminarizing flow in the stationary tube case, both the turbulent kinetic energy and the temperature variance remain over the pipe cross section, resulting in the suppression of an attenuation in heat transfer performance. In other words, an increase in tube rotation suppresses laminarization of gas flow.

Analysis of Enhanced Heat Transfer Performance Through Duct with Constant Wall Temperature

2012 ◽  
Vol 26 (3) ◽  
pp. 472-475 ◽  
Author(s):  
Yan He ◽  
Hongcai Zheng ◽  
Qian Zhang ◽  
Yuanzheng Tang ◽  
Jianghui Zhang
Keyword(s):  
Heat Transfer ◽  
Wall Temperature ◽  
Heat Transfer Performance ◽  
Transfer Performance ◽  
Enhanced Heat Transfer ◽  
Constant Wall Temperature
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