Analysis of evaporative heat transfer of miniature heat pipes for cooling electronic devices Fan Chunli 1 Qu Wei 2 Yang Li 1 Hua Shunfang 1 Ma Tongze 2 "l. Teaching and Research Section 307, School of Power Engineering, Naval Engineering University, Wuhan 430033, Hubei, China; Institute, Beijing 100080 research shows that the effect of thin liquid film on the heat transfer of the heat pipe is much greater than that of the thick liquid film; the solid-liquid contact angle gradually decreases along the axis in the evaporation section; the heat transfer calculation of the heat pipe with processing corners At the time, there is a restriction relationship between the minimum meniscus radius and the processing fillet, and it cannot be simply replaced by processing fillet.
Micro heat pipe is an emerging technology developed along with the development of microelectronic technology. With the increase of the number of circuits in the computer chip, it becomes more and more difficult to dissipate heat. For example, the heat flux generated by the chip of the host computer and the main computer has reached 60W / cm2, and in 2000 it has exceeded 100W / cm2 .In addition to the requirement of the highest temperature on the chip, there is also a higher requirement for the uniformity of temperature. Therefore, the heat dissipation of electronic devices has become a very important technology in the development and development of electronic products. Its heat dissipation performance directly affects the final electronics. Product cost, reliability and performance. As a promising technology, micro heat pipes are being used in electronic devices to obtain a high heat extraction rate and temperature uniformity.
Since Cotter proposed the concept of micro heat pipes in 1984, people have made a lot of theoretical summaries of micro heat pipes. The micro heat pipes are composed of an evaporation section, an adiabatic section and a condensation section. The liquid film distribution is shown. Assumptions for the evaporation section: â‘ The steam temperature is at the saturation temperature of the working medium, and there is no gradient along the axis.
â‘¡The heat pipe is in a stable working state. â‘¢ At any cross-section in the axial direction, the radius of the meniscus in each corner area is a constant value, and only changes in the axial direction.
â‘£The temperature of the wall of any section of the heat pipe is uniform, and only changes in the axial direction.
Temperature and steam temperature, T is the average of steam temperature, liquid temperature and vapor-liquid interface temperature, is surface tension, K is the meniscus curvature, and p (i is the detachment pressure.
In the equilibrium film area, the detachment pressure plays a dominant role, and T. is taken as the saturation temperature Tsat of the working medium. From qm (. = 0, the thickness of the liquid film in the boundary line area can be obtained where A is the Hamakei constant and Tw is the heat pipe temperature.
Through the analysis of flow and heat transfer, the second order differential equation 141 for solving the thickness of the thin liquid film is: Pi is the viscosity of the liquid. Because the thickness of the liquid film in the thin liquid film area of ​​evaporation is very small, the heat transfer heat flow of the liquid film is (surface thermal resistance Ri): the schematic diagram of the liquid film distribution on the wall surface can be imagined, when the heat pipe reaches the maximum heat transfer capacity, the processing corners of the end of the evaporation section The upper end of a layer of evaporating liquid film is exactly the transition position of the balance film area and the evaporating film area.
For example, the micro-liquid film area includes two parts, and one part is a balanced film area. The thickness of this part of the liquid film is very small. Due to the influence of the pressure release, the evaporation of the liquid film is suppressed, and the evaporation amount is almost the thermal conductivity of ki.
Equation (2) can be solved by the fourth-order Runge-Kutta method.
The value of T * in each step can be obtained by coupling iterations of equation (1) and equation (3), so that the circumferential distribution of T * and heat flux density in the thin liquid film can be obtained.
2 Meniscus area The evaporative heat flux density of the meniscus area at any point in the circumferential direction is
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