Research on a Simplified Model of an Aluminum Vapor Chamber in a Heat Dissipation System

With the rapid increase of power densities of electronic components, the traditional heat dissipation method of air forced convection has reached a heat transfer limit. As efficient phase change heat exchangers, vapor chambers have become an important guarantee for the development of high-power electronic components. Aluminum vapor chambers have become the future development trend because they are more lightweight and less expensive. In order to study the suitable simplified model of the aluminum vapor chamber in the radiating system, the testing system is established to test the thermal characteristics of the vapor chamber. First, six simplified models of the vapor chamber are proposed. Then, the thermal characteristics of the simplified models are simulated by STAR CCM+ software. Next, the error of the thermal resistance of the simplified model and the real vapor chamber is analyzed. Finally, a most suitable simplified model is obtained in the cooling system.

Experimental Investigation of an R134a Loop Thermosiphon for Shaft Cooling

Shaft cooling based on a loop thermosiphon is an ideal method for cooling of motorized spindles since it transfers heat with high efficiency and does not require an external power supply. In this study, an experiment was conducted on an R134a single-loop thermosiphon when the evaporation and condensation sections were on the same pipe. Results indicated that the single-loop thermosiphon was still operational with a minimum average thermal resistance of 0.51 W/°C when the filling ratio (FR) was 40%. The temperature distribution of the test specimen was determined predominantly by the amount of heating power, and not the mode. The optimum liquid filling ratio was 40% – 60% under this special condition, and both the thermal resistance and the heat transfer limit increased with the increase of FR in this range. The maximum temperature of the 150SD motorized spindle decreased 29% with the use of the R134a shaft cooling structure.

Flow Visualization inside Thermosyphon for Measuring Heat Transfer Limit

Heat pipe is a highly effective passive heat transfer device using phase change within small temperature difference. It is noted that heat pipe should be operated under heat transfer limit for practical heat pipe heat exchanger applications. The measurement in local and overall heat transfer coefficient is significant to anticipate the heat transfer limit. The wall temperatures and inner working fluid temperatures were measured to determine the heat transfer coefficient. The adiabatic part with transparent Pyrex glass was visualized to understand flow behaviors inside the thermosyphon. The dynamic behaviors of condensed working fluid were visualized for the specific tilted angle and power inputs at pseudo steady-state. At low heat input of 250W, the thin condensed liquid film is observed to be returned from condenser to evaporator. With increasing heat input of 500W, the nucleate boiling starts to occur in evaporator. More activated vapors turn to make wavy motion in free surface of the returned condensed liquid film which is thickened. In power input of 1,250W, the vigorous flow motion happens periodically and the interaction between vapor and liquid bursting reaches a maximum heat transfer which is led to the heat transfer limit in the thermosyphon. Over heat transfer limit (2,000 and 2,500W), the overall heat transfer is decreased when the degree of bursting motion between vapor and liquid is gradually reduced.

Wettability and nanostructure effect on oscillating heat pipes

As electronics technologies rapidly develop with a demand for more power and miniaturization, effective thermal management of these systems becomes much more important. The oscillating heat pipe (OHP) is a promising highly efficient heat transfer device that is great for high heat flux applications common in the electronics industry. In the current investigation, the wettability effect on the heat transfer performance of OHPs has been conducted. 1). The overall performance of configuration of hydrophilic evaporator/ hydrophobic condenser and hydrophobic evaporator/ hydrophilic condenser was worse than the nontreated OHP, however; the oscillations were much damper when comparing the amplitudes. 2). High oscillating motion occurs in the OHP with the hydrophilic surface while low oscillating motion occurs in the untreated OHP. 3). A mathematical model shows that contact angle increases the oscillating motion decreases. 4). A theoretical model predicting operating limit is developed. Results show that radius and charging ratio has a large effect on the maximum heat transfer limit. Working fluids changes the operating limit.

Experiment Study on Improved Closed Loop Pulsating Heat Pipe with Silver/Water Nanofluid

Start up and heat transfer performances of improved closed loop pulsating heat pipe (ICLPHP) charged with water and silver/water nanofluid, respectively, were investigated experimentally with angles of 90° and 60°. Both the average evaporator wall temperature and the overall thermal resistance of the ICLPHP with different working fluids and at the volume filling ratio of 35% were tested and compared. Experimental results showed that nanofluid caused different thermal performances of ICLPHP. Within the experiment range, silver/water nanofluid can improve operation stability and heat transfer limit and reduce starting power compared with water. With high heating power, thermal resistance of nanofluid was lower than that of water. With inclination of 60°, ICLPHP with nanofluid operated better and reduced sensitivity of inclination.

Analysis on the Selection of Working Fluid in the Small Diameter Gravity Heat Pipe - Based on a New Passive Technology

This paper discusses the suitable working fluid applying in small diameter (millimeter scale) gravity heat pipe theoretically. The working temperature, characteristics of material, heat transfer limit and thermal physical properties of working medium of the heat pipe were studied. It is concluded that each aspect capability of heat transfer of R717 is excellent, but its working pressure is a bit high; the synthesized capability of R134a is relatively ideal and can be chosen as the working medium.