Search

The cooling system is the auxiliary equipment that consumes the most energy in a data center, accounting for about 30 to 50% of the total energy consumption. In order to effectively reduce the energy consumption of a data center, it is very important to improve the heat exchange efficiency at the chip level. Compared with air cooling, single-phase cold plate liquid cooling, and immersion liquid cooling, the flat loop heat pipe (FLHP) is considered to be a better chip-level cooling solution for data centers. It has extremely high heat transfer efficiency and heat flux variability, and it can avoid the operation risk caused by liquid entering the server. In this paper, a FLHP with an evaporator designed with a "Tesla valve" flow channel configuration is developed. Experiments on the FLHP are carried out, focusing on the installation angles and cooling condition factors. The results show that an inclination angle of 20° is the critical point of the influence of gravity on the performance of the FLHP; to ensure good operation of the FLHP, the installation angle should be greater than 20°. The equivalent heat transfer coefficients of the FLHP condenser under different cooling conditions are calculated. It is found that water cooling can provide higher cooling heat transfer coefficients with lower energy consumption and operating noise. Additionally, the heat transfer limit, operating temperature uniformity, and start-up stability of the FLHP are significantly improved under water cooling conditions. The maximum heat load of the FLHP is up to 230 W, and the temperature difference of the evaporator surface can be controlled within 0.5 °C, under 20 °C water cooling. Finally, using the FLHP for thermal management of the chip, its heat transfer efficiency is 166 and 41% higher than that of air cooling and water cooling, respectively. [ABSTRACT FROM AUTHOR]

This paper aims to deeply explore the influence of different inclination conditions on the heat transfer characteristics and broaden the application scene of a pulsating heat pipe. A test device for the heat transfer performance of a pulsating heat pipe under different inclination angles is designed and built. Under the condition of 70% liquid filling rate, ethanol and HFE-7100 are selected to carry out the experimental test with heating power of 40–140 W and dimensionless thermal difference of 0–0.56. The heat transfer performance, the temperature in the evaporation section and the internal pressure fluctuation of the pulsating heat pipe were experimentally studied. The results show that under the condition of uniform heat flux, for ethanol working medium, when the pulsating heat pipe is heated at 40 W, the operating thermal resistance varies significantly with different installation angles. At this time, the operating thermal resistance of a pulsating heat pipe with installation angles of 45°, 70°, 90° and D90° is 1.38 °C/W, 1.60 °C/W, 1.73 °C/W and 2.07 °C/W, respectively. With the increase in installation angle, the operating thermal resistance also increases gradually, reaching the maximum at 90°. At low heating power, the effect of the installation angle on the ethanol working medium is significantly greater than that of HFE-7100 working medium. The HFE-7100 working medium showed lower operating thermal resistance at low heating power, but with the increase in heating power, the operating thermal resistance of the two working medium gradually approached a 70% filling rate. Under non-uniform heating conditions, when HFE-7100 is used as a working fluid, the operating thermal resistance of a pulsating heat pipe under different heating power was lower than that of the ethanol working medium. The operating thermal resistance is less affected by the installation angle, and the overall heat transfer performance is better. The phenomenon in which the ethanol working medium is obviously affected by the installation angle can be improved by non-uniform heating conditions. For ethanol working medium, when the dimensionless heat difference reaches 0.33 under the condition of a 45° installation angle, the average temperature fluctuation in the evaporation section appears gentle. At this installation angle, the internal working medium of the four elbow pulsating heat pipe devices used in this research more easily forms a cycle in the pipe than the 90° installation angle. [ABSTRACT FROM AUTHOR]

Currently, the growing need for efficient refrigeration resources in the industrial sector has led to an increasing interest in finding technologies with a higher heat removal potential and better cooling performance. Along these lines, two-phase liquid cooling appears to be a very interesting solution, with the CLTPT (closed-loop two-phase thermosyphon) being one of the leading alternatives. Most works in the scientific literature study loop thermosyphons that work in flow boiling conditions in steady state. The present paper analyzes the transient thermal behavior of a pool boiling CLTPT gravitational channel as a passive cooling system using NOVEC 649 as working fluid. The evaporator works with two submerged cylindrical heaters that represent different heat sources located in different positions. The initial transient behavior and consequent instabilities of a laboratory-scale facility were studied, followed by a stability analysis for various power inputs. Parameters such as temperature and pressure along the experimental setup were monitored, and the effects of internal pressure and room conditions were also tested. The results show some instabilities in the process to start the flow circulation and a relative stability and quick adaptation to changes when circulation is reached. The temperature in the evaporator chamber was highly homogeneous during the whole process; however, the temperature changes in the riser and the loop top were delayed with respect to the evaporator zone. The analysis shows several pressure and temperature raises before the vapor flux reaches the condenser. When the flow circulation is established, the system becomes highly stable and thermally homogeneous, decreasing the thermal resistance when increasing the power input. The stability analysis also showed that, when the system reaches the steady state, the changes in the power input produce a transient increase in the pressure and temperature of the fluid, followed by a quick decrease of the previous steady state values. The heat transfer analysis in the evaporator shows a higher heat flux on the upper heater caused by the buoyancy flow that rises from the lower heater. It was also observed that the lower heater reaches the CHF point with a lower heat flux. [ABSTRACT FROM AUTHOR]

Coolants are widely used to dissipate grinding heat in conventional grinding. This process, however, is not satisfactory as coolants often lose efficacy in grinding due to film boiling and can result in adverse health and environment effects. The present paper put forward the concept of a rotating heat pipe grinding wheel, attempting to reduce or eliminate the coolant amount and realize green machining. The heat transfer performance of rotating heat pipe grinding wheel was studied by using volume of fluid method in ANSYS/FLUENT. The influence of the input heat flux, filling ratio and rotational speed were investigated by a simulation method. Results show that the appropriate heat flux range for the rotating heat pipe grinding wheel was from 2000 to 100,000 W/m2, the ideal filling ratio was 50% and the rise of the rotational speed turned out to weaken the heat transfer coefficient. Finally, dry grinding experiments on Ti-6Al-4V were performed and the temperatures in both the rotating heat pipe and the grinding contact zone were monitored. The new designed rotating heat pipe grinding wheel showed a good prospect for application to green grinding of difficult-to-cut materials. [ABSTRACT FROM AUTHOR]