Enhanced evaporation heat transfer in triangular grooves covered with a thin fine porous layer

doi:10.1016/S1359-4311(01)00044-8

Applied Thermal Engineering

Volume 21, Issue 17, December 2001, Pages 1721-1737

https://doi.org/10.1016/S1359-4311(01)00044-8 Get rights and content

Abstract

Evaporation heat transfer in a triangular groove is enhanced by a thin fine porous layer on the groove surface. This not only improves the capillary force but also extends the evaporating surface with high heat transfer performance. The enhancement mechanism is explained and an analytical model is developed to predict evaporation heat transfer performance in a triangular groove formed by trapezoidal fins covered by a thin fine porous layer. The total heat transfer coefficient decreases with increasing liquid meniscus radii, increases with increasing of the half groove angle and the half width of the fin top surface. With receding of the liquid meniscus in the groove covered by a thin porous layer, the evaporation heat transfer performance is improved significantly. A comparison between evaporation heat transfer in triangular grooves with and without a thin porous layer, at the same conditions, shows the former is much better than the latter although the latter has a small area with a heat transfer coefficient as high as $10^{8} W m^{−2} K^{−1}$ . In the calculation range, evaporation heat transfer in the groove covered by a thin porous layer is three to six times higher than that in the groove without a porous layer.

Introduction

Contemporary technology is characterized by the desire to increase the power of a device while making it smaller and smaller. Recent developments in aerospace, energy, optics, electronics and electrical engineering have led to demands for high performance heat transfer devices in order to transport more energy per unit area and have wider operational ranges. For instance, the development of modern electronic chips has led to a need for high performance heat sinks to remove energy at very high heat fluxes, 1 $KW cm^{−2}$ and higher. Some compact and miniature heat exchangers are designed to transport high density heat flow under limitations of area and temperature difference. In some laser diode arrays, heat fluxes in heat sources are very high and they need to be cooled. The challenge and difficulty of these heat transfer problems are that they need to be dealt with very little temperature drops allowed.

There are several methods to control and remove the high heat flux energy generated in the above devices like forced convection in micro-channels, pool and flow boiling with enhanced surfaces and liquid jet impingement et al. Among them, heat pipes are a passive cooling method that can effectively transport the heat with a small temperature difference, and, as a result, they are widely used in many areas of the industry. A heat pipe is a closed evaporation-condensation two phase system in which heat is transported by vapor–liquid phase change and fluid circulation is activated by capillary force. Grooved heat pipes are one of the most common types of heat pipes and widely applied in aerospace, energy and electronics industries.

As early as the begin of 1970s, Edward and coworkers [1], [2] investigated evaporation and condensation phenomena in circumferential grooves on horizontal tubes and solved the two dimensional equations of capillary flow in triangular grooves by the Galerkin boundary method. A lot of works on evaporation phenomena in grooves have been carried out in the last decade. Xu and Carey [3] presented an analytical model for predicting the heat transfer characteristics of film evaporation on a microgroove surface and its comparison with the limited experimental data. Stroes et al. [4] experimentally investigated the capillary forces in channels with different sections. Stephan and Busse [5] numerically analyzed the heat transfer coefficient of grooved heat pipe evaporator walls and indicated that the assumption of an interface temperature equal to the saturation temperature of the vapor can lead to a large over predication of the radial heat transfer coefficient. Khrustalev and Faghri [6] developed a mathematical model for evaporation heat transfer through thin liquid films in the evaporators of grooved heat pipes. Their results demonstrated the importance of the surface roughness and interfacial thermal resistance. Ha and Peterson [7], [8], [9] investigated evaporation heat transfer and capillary flow in micro grooves. A non-dimensional expression was developed as a function of just one parameter for predicting the characteristics of the capillary flow. Ma and Peterson [10], [11] analyzed the temperature variation and heat transfer in triangular grooves with an evaporating film and experimentally investigated the maximum heat transport in triangular grooves. The results indicated that there existed an optimum groove configuration. The heat transfer capability of grooved heat pipes is mainly limited by the minimum curvature of the liquid menisci in grooves. In order to improve the operating performance and heat transfer capacity of the grooved heat pipes for high heat flux applications, the first thing one must do is to enhance evaporation heat transfer within the grooves. Herein evaporation heat transfer in a groove is analyzed and shown to be significantly enhanced by covering the surface with a thin fine porous layer.

Section snippets

Theoretical analyses

Evaporation heat transfer in a triangular groove occurs on a meniscus surface formed in grooves. Most investigators have focused evaporation on menisci formed in grooves or micro grooves with extended thin film, as shown in Fig. 1(a). Intensive evaporation heat transfer occurs on the thin film region that extends from the intrinsic meniscus. However the extended thin film in the grooves with different sections is a very small portion of the total surface of the grooves. A large fraction of the

Results and discussion

Evaporation heat transfer performance in triangular grooves covered with a thin fine porous layer is estimated for different geometrical parameters of grooves and menisci using the above method. Water is the working fluid, the vapor temperature T_v is 373.13 K and the root wall temperature T_w is 374.13 K. The parameters of the thin fine porous layer are: its thickness δ_w=0.1 mm, permeability $K=10^{−8} m^{−2}$ and effective thermal conductivity k_eff=20 W m⁻¹ K⁻¹. The parameters of the aluminum alloy fin

Conclusions

Evaporation heat transfer in triangular grooves is analyzed. Based on the fact that the extended thin liquid film region where very high heat transfer rates occur is only a very small portion of the total groove surface, while the remaining surface has low heat transfer rates, a thin fine porous layer is suggested to enhance the evaporation heat transfer in grooves, especially in situations when high heat fluxes are high. This method not only improves the capillary force for liquid supply but

Acknowledgements

The authors would like to acknowledge the support of Defense Advanced Research Projects Agency, DOD.

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Enhanced evaporation heat transfer in triangular grooves covered with a thin fine porous layer

Abstract

Introduction

Section snippets

Theoretical analyses

Results and discussion

Conclusions

Acknowledgements

Int. J. Heat Mass Transf.

Capillary flow in triangular grooves

ASME J. Appl. Mech.

Film evaporation from a micro-grooved surface – an approximate heat transfer model and its comparison with experimental data

AIAA J. Thermophys. Heat Transf.

An experimental study of the capillary forces in rectangular channels vs. triangular channels

ASME NHTC

Heat transfer during evaporation on capillary grooved structures of heat pipes

J. Heat Transf.

The interline heat transfer of evaporating thin films along a micro grooved surface

ASME J. Heat Transf.

Capillary performance of evaporating flow in micro grooves: an approximate analytical approach and experimental investigation

ASME J. Heat Transf.

Capillary performance evaporating flow in micro grooves: an analytical approach for very small tilt angles

ASME J. Heat Transf.