Recent Advances in Loop Heat Pipes with Flat Evaporator

The focus of this review is to present the current advances in Loop Heat Pipes (LHP) with flat evaporators, which address the current challenges to the wide implementation of the technology. A recent advance in LHP is the design of flat-shaped evaporators, which is better suited to the geometry of discretely mounted electronics components (microprocessors) and therefore negate the need for an additional transfer surface (saddle) between component and evaporator. However, various challenges exist in the implementation of flat-evaporator, including (1) deformation of the evaporator due to high internal pressure and uneven stress distribution in the non-circular casing; (2) heat leak from evaporator heating zone and sidewall into the compensation chamber; (3) poor performance at start-up; (4) reverse flow through the wick; or (5) difficulties in sealing, and hence frequent leakage. This paper presents and reviews state-of-the-art LHP technologies; this includes an (a) review of novel manufacturing methods; (b) LHP evaporator designs; (c) working fluids; and (d) construction materials. The work presents solutions that are used to develop or improve the LHP construction, overall thermal performance, heat transfer distance, start-up time (especially at low heat loads), manufacturing cost, weight, possibilities of miniaturization and how they affect the solution on the above-presented problems and challenges in flat shape LHP development to take advantage in the passive cooling systems for electronic devices in multiple applications.

Ensuring the required law implementation accuracy of the transmission clutches control by regulating the working fluid volume in the hydraulic cylinder compensation chamber

Introduction (problem statement and relevance). Testing the vehicle automatic transmission and shifting gears without interrupting the power flow undesirable dynamic phenomena were revealed. The occurrence of “parasitic” centrifugal pressure in the clutch boosters was observed which resulted in self-activation of the control element, clamping the discs friction linings, which made the hydraulic cylinder emptying and piston removing from the package disks complicated. As a consequence of it there occurred a comfort decrease in the vehicle, and in some cases, the destruction of friction clutches. It was found that the reason for this occurrence was the filling degree instability of the compensation and piston chambers of the hydraulic cylinder clutch, which was not taken into account by the existing calculation and design methods under various initial conditions.The purpose of the study was to improve the implementation accuracy of the required control law of the transmission clutches by purposeful regulating the working fluid volume in the compensation chamber of the hydraulic cylinder clutch.Methodology and research methods. A road test technique was proposed for identifying and reproducing the conditions of the dynamic phenomenon manifestation. The developed mathematical model of the piston stroke made it possible to assess the dependence of the implementation quality of the required clutch control law when individual gears were engaged on the following parameters: the filling degree of the compensation chamber; features of solenoid valves operation; the stiffness of the return spring; the number of clutch friction pairs.Scientific novelty and results. On the experimental research basis of the worked out design and the use of scientifically grounded technical solutions the dynamic effect manifestation of the unbalanced “parasitic” centrifugal pressure rise was excluded.Practical significance. The developed and implemented technical solutions for stabilizing the pressure in the compensation chamber made it possible to ensure the required quality of gear shifting when limiting the dynamic and thermal loading of the friction discs, which made it possible to ensure the required level of dynamic characteristics and comfort ability of the product when shifting gears.

Experimental Study of a Loop Heat Pipe with Direct Pouring Porous Wick for Cooling Electronics

A pouring silicate wick was manufactured to explore the influence of process and physical properties on the production and performance of loop heat pipes (LHP). This paper theoretically analyzed the advantages of pouring porous wick and introduced the technology of pouring silicate directly on evaporator. Based on this, the heat transfer performance of copper-methanol LHP system with pouring porous wick was tested under different positions. The results showed that with the input of multiple heat sources, the LHP could start up and maintain a stable temperature from 40 W to 160 W. When the vapor grooves were located above the compensation chamber, it was difficult to start up positively. By adding gravity assistance, the system could obtain more stable liquid supply and vapor flow, so as to realize start up. In the variable heat load test, the LHP showed good adaptability to the change of heat load. The thermal resistance of the system decreased with the increase of heat load. The thermal resistance of the evaporator almost unchanged and was always lower than 0.05 °C/W, which indicated that the pouring porous wick in the evaporator had good heat load matching.

Modeling of Loop Heat Pipe Thermal Performance

The present work deals with the heat transfer performance of a copper-water loop heat pipe (LHP) with a flat oval evaporator in steady-state operation. Modeling the heat transfer in the evaporator was particularly studied, and the evaporation heat transfer coefficient was determined from a dimensionless correlation developed based on experimental data from the literature. The model was based on steady-state energy balance equations for each LHP component. The model results were compared to the experimental ones for various heat loads, cooling temperatures, and elevations, and a good agreement was obtained. Finally, a parametric study was conducted to show the effects of different key parameters, such as the axial conductive heat leaks between the evaporator and the compensation chamber cases, the capillary structure porosity and material, and the groove dimensions.