Two-phase Heat Transfer Devices Research Articles

Review of “cold shock” cases in operation of loop heat pipes and related thermal instabilities

Loop Heat Pipes (LHP) are passive two-phase heat transfer devices, driven by capillary pumps that source energy for circulation of working fluid solely from the heat source. They have a proven track record in spacecraft thermal control as well as terrestrial applications. However, LHP may experience partial or complete wick dryout and thermal instabilities at rapid power-up steps or (and) rapid condenser cooling conditions change due to a rapid inflow of subcooled liquid from the condenser into the evaporator. Such cases do not have very wide coverage in literature and are usually presented as a little nuisance in testing of a particular LHP, for which some design workaround was found. Oftentimes suggested solutions trade performance or other design merits for robustness of operation. This article aims to review such case studies where phenomena similar to the described “cold shock” affected LHP performance. The aforementioned instabilities become even more pronounced in grid-like multievaporator LHP having multiple parallel connections between capillary pumps and compensation chambers, as heat and mass transfer between those elements becomes less definite and can eventually increase the heat leak into compensation chambers. Vulnerability of LHP with distributed network of evaporators to seemingly “safe” transient regimes fosters deeper analysis of the “cold shock” phenomenon. With comprehensive definition to the problem and a list of research questions, technical measures for getting rid of it in perspective LHP designs can be foreseen.

Experimental study of a bilayer ceramic wick for the evaporator of a two-phase heat transfer device

In order to enhance the thermal performance of two-phase fluid loop devices, the purpose of this study is to investigate the benefit of using a bilayer wick in a capillary evaporator.In a homogenous wick, the porous structure characteristics present an intrinsic competition between increasing the capillary forces and reducing the pressure drop.Our investigation focuses on studying heat and mass transfer in uniform and bilayer ceramic wicks with the same porosity. The findings highlight the interest of a bilayer ceramic wick with a pore diameter gradient. This configuration exhibits heightened capillary pumping without compromising liquid pressure drop, leading to improved thermal performance in the evaporator.Our results demonstrate that opting for a bilayer ceramic with a pore diameter gradient can save up to 25 °C at the casing temperature and reach up to 1430 W/m²K−1 as heat transfer coefficient for a 70 W heat load value.

Advanced loop heat pipe application for cooling high power LED lights

The ever-increasing power density of electronic devices fosters development of cooling systems. Among them, two-phase heat transfer devices in general and Loop Heat Pipes (LHP) in particular offer advantages by being completely passive while being able to cope with high heat fluxes. However, LHP manufacturing and especially integration is complicated by the loop architecture, which is not found in conventional Heat Pipes (HP). In an attempt to fit LHP layout into HP-like housing, a novel Heat Loop Pipe (HLP) device was designed, manufactured and tested in application to cooling a portable high-power (80 W) LED luminaire with a focusing reflector. The HLP utilizes co-axial layout of vapor and liquid lines and an additional wick in the condenser. The butane-charged, 350 mm-long HLP showed unexpectedly high resistance in the adverse orientation (evaporator above the condenser), which was an order of magnitude higher than in reflux orientation (0.8 K/W vs. 0.03 K/W). A possible explanation of this behavior in terms of vapor superheating in the evaporator channels is given. Analysis of the test results with help of p-T diagram justifies this assumption. Finding the root cause for such behavior requires further study.

Design and experimental validation of a high capacity loop heat pipe for avionics cooling

With the rapid development of modern aviation industry, the power of avionics has been increasing continuously, bringing great challenge to thermal management. As an efficient two-phase heat transfer device, loop heat pipe (LHP) holds great application potential in avionics cooling. To address the issue of inadequate heat transfer capability, a set of kW-class LHP was specially designed and experimentally validated considering the airborne environment. A detailed introduction covering the entire design process was presented, in which some targeted design criteria were provided. Ammonia was chosen as the optimal working fluid, and stainless steel envelope with a nickel wick was adopted. Experimental validation showed that, at a heat sink temperature of 20 ℃, the designed LHP could achieve a maximum heat transfer capacity of 1021 W, confirming the efficacy of the design method. The average heat flux at the evaporator could reach 12.04 W/cm2. The LHP exhibited a maximum and minimum thermal resistance of 0.16 and 0.054 ℃/W, respectively, achieved at heat loads of approximately 50 W and 600 W. The maximum temperature fluctuation at the center of the saddle surface was no more than ±1.40 ℃. A method for estimating the heat leak between the evaporator and the compensation chamber was also developed, and the maximum heat leak rate was calculated to be approximately 5.2%. Moreover, the designed LHP also manifested excellent startup performance and operation stability.

Hydrophilicity degradation and steam-induced rewetting during capillary-fed boiling

Capillary-fed evaporation/boiling is a crucial process that occurs within two-phase heat transfer devices. Herein, we delve into the capillary-fed boiling performance of a grooved wick with an upper width of 200 μm and a depth of 150 μm fabricated by ultrafast laser micromachining and observe a substantially 51.8% reduction in critical heat flux (CHF), from 145.0 ± 3.3 W/cm2 to 70.1 ± 2.9 W/cm2, following five boiling cycles. Comprehensive experimental investigations are conducted to gain insights into the underlying mechanisms. Our analyses confirm that the diminished CHF arises from the hydrophilicity degradation of the evaporator, attributed to the surface adsorption of airborne organics after the dry-out. The superhydrophilic evaporator becomes highly hydrophobic with static contact angles (CAs) greater than 140° after five boiling cycles. Liquid wicking in subsequent boiling tests initiates through a steam-induced rewetting process. Moreover, we present compelling evidence that the undesired hydrophilicity degradation is effectively mitigated on nanoporous surfaces. Consequently, a microgroove-nanoparticle composite wick successfully retains its original boiling performance during repeated boiling tests. These findings enhance the understanding of capillary-fed boiling performance and offer promising avenues for the design of high-performance wicks tailored for ultrathin two-phase heat transfer devices.

Biomimetic Copper Forest Structural Modification Enhances the Capillary Flow Characteristics of the Copper Mesh Wick

In a two-phase heat transfer device, achieving a high capillarity of the wick while reducing flow resistance within a limited space becomes the key to improving the heat dissipation performance. As a commonly used wick structure, mesh is widely employed because of its high permeability. However, achieving the desired capillary performance often requires multiple layers to be superimposed to ensure an adequate capillary, resulting in an increased thickness of the wick. In this study, an ultra-thin biomimetic copper forest structural modification of copper mesh was performed using an electrochemical deposition to solve the contradiction between the permeability and the capillary. The experiments were conducted on a copper mesh to investigate the effects of various conditions on their morphology and capillary performance. The results indicate that the capillary performance of the modified copper mesh improves with a longer deposition time. The capillary pressure drops can reach up to 1400 Pa when using ethanol as the working fluid. Furthermore, the modified copper mesh demonstrates a capillary performance value (ΔPc·K) of 8.44 × 10−8 N, which is 1.7 times higher than that of the unmodified samples. Notably, this enhanced performance is achieved with a thickness of only 142 μm. The capillary limit can reach up to 45 W when the modified copper mesh is only 180 μm. Microscopic flow analysis reveals that the copper forest modified structure maintains the original high permeability of the copper mesh while providing a greater capillary force, thereby enhancing the overall flow characteristics.

Comparative analysis of CLPHP working with binary mixtures in different concentrations

The advanced technologies in design and manufacturing of electronic components have resulted considerable heat flux through component miniaturization. For highly performing microelectronic devices production, thermal management has the priority. Other applications also like material science, fabrication technology, solar applications etc. which directly impact cost, reliability and performance. When it comes to overcome these problems efficient cooling approaches development and use will come into play. The best solution for this is Pulsating heat Pipe (PHP). A simple PHP is a two-phase heat transfer device consists of a tube with a suitable working fluid, along with an evaporator (heating)section, adiabatic section and a condenser (cooling)section. In the present work multi turn Closed Loop Pulsating Heat Pipe (CLPHP) is chosen with capillary dimensions. The experimentation is carried out with different working fluids viz. water, water-based mixtures (75%-25%) at different volumetric fill ratios 50%,60% and 75%. The heat inputs of20W, 40 W, 60 W, 80 W, 100 W applied. At various heat inputs for all the working fluids the variation of thermal resistance is plotted and observed that upon increasing heat input, thermal resistance of fluid decreases. Comparative study is also presented for same experimental setup when the binary mixtures are considered in 50%-50% as mixture concentration. This gives the better understanding of Closed Loop Pulsating Heat Pipe performance when operated at different fill ratios and concentrations for different working fluids considered.

Estimation of the local instantaneous heat flux inside a pulsating heat pipe for space applications

Pulsating Heat Pipes (PHP) are promising and effective passive two-phase heat transfer devices in terms of high heat transfer capability, efficient thermal control, adaptability and low cost and therefore they have been extensively studied in the last years. Many authors have estimated the heat fluxes at the evaporator and at the condenser area only in terms of the average values based on first principle considerations. In the present study the application of an inverse analysis technique to experimental infrared temperature data is proposed to investigate the local convective heat flux for forced convection flow inside these devices along the adiabatic zone. A PHP specifically designed to be hosted on board the heat transfer host of the International Space Station was tested in microgravity during the 67th Parabolic Flight Campaign organized by the European Space Agency. The device consists of an aluminium tube closed in a loop with 14 turns in the evaporator section, 3 mm inner diameter, half filled with FC-72 fluid. The external temperatures of the device are measured in the adiabatic zone with a high-speed infrared camera (50 Hz, 1280x1024 pixels). The images are thereafter post-processed and adopted as input data for the solution of the inverse heat conduction problem in the pipe wall (Tikhonov regularisation method) to extrapolate time-varying local heat fluxes on the tube internal surface in contact with the fluid.

Polymer and Composite Materials in Two-Phase Passive Thermal Management Systems: A Review.

The application of polymeric and composite materials in two-phase passive heat transfer devices is reviewed critically, with a focus on advantages and disadvantages of these materials in thermal management systems. Recent technology developments led to an increase of the power density in several applications including portable electronics, space and deployable systems, etc., which require high-performance and compact thermal management systems. In this context, passive two-phase systems are the most promising heat transfer devices to dissipate large heat fluxes without external power supply. Usually, heat transfer systems are built with metals due to their excellent thermal properties. However, there is an increasing interest in replacing metallic materials with polymers and composites that can offer cost-effectiveness, light weight and high mechanical flexibility. The present work reviews state-of the-art applications of polymers and composites in two-phase passive thermal management systems, with an analysis of their limitations and technical challenges.

The role of the electric field in the departure of vapor bubbles in microgravity

We present the results of a study aimed at investigating the effects of electric fields on quasi-static bubble departure dynamics during pool boiling of perfluorohexane (FC-72) in microgravity conditions. Analysis was performed through an alternative formulation of the bubble momentum balance in which the contribution of non-uniform electric stress distributions at the bubble interface can be quantified through high-speed video measurements without having to numerically solve the laws of electrostatics. Data used in this study were obtained in the scope of the Multiscale Boiling Project, which included advanced single bubble growth experiments performed aboard the International Space Station. Our results confirm that bubble departure counterintuitively begins before the force resulting from electric stresses starts to pull the bubble up from the wall. When this occurs, it is shown that the shrinking process of the contact line accelerates, in agreement with known theoretical results. It is concluded that the electric force is essentially determined by the electric stress distribution at the bubble cap above the contact area. Furthermore, we show that the electric stress at the bubble interface is also responsible for the increase in bubble internal overpressure, which explains the early departure of the bubble while increasing the intensity of the electric field. The results of this study provide an important step in achieving a more comprehensive understanding of the bubble behavior at the heated surface in the presence of an electric field, which is essential to optimally design electrodes and two-phase heat transfer devices for future space applications.

Two-phase heat transfer devices for passive cooling of electric and hybrid aircraft onboard equipment

Two-phase heat transfer devices for passive cooling of electric and hybrid aircraft onboard equipment

Two-phase heat transfer devices for passive cooling of electric and hybrid aircraft onboard equipment

Two-phase heat transfer devices for passive cooling of electric and hybrid aircraft onboard equipment

Parametric Study on Ammonia-Based Loop Heat Pipe

The loop heat pipe (LHP) is a two-phase heat transfer device used for thermal management in applications like space, mobile, laptop, electronic cooling. In the present study, modeling and parametric analysis of LHP was done by applying energy balance approach at different sections using Dev-C++ software. The effects of various parameters like sink temperature (5 °C, 10 °C, 22 °C), condenser effectiveness (0.4–0.9), vapor line diameter (0.0043 m, 0.0053 m, 0.0063 m), wick thickness (0.0073 m, 0.0048 m) and wick porosity (40%, 60%, 80%) were studied on the performance of LHP. The results obtained were validated with the results published in the literature. With increase in condenser effectiveness from 0.40 to 0.90, steady state operating temperature got decreased by 4.5 °C at a heat load of 600 W. The studies on wick porosity revealed that it has significant effect on the heat leak from evaporator to compensation chamber and the lower porosity leads to higher heat leak. From the analysis, an optimum performance of LHP was observed with 60% wick porosity, 0.0073 m wick thickness, 5 °C sink temperature, vapor line diameter of 0.0053 m and condenser effectiveness as 0.9 among the various parameters under investigations.

Capillary Jet Loop performance in parabolic flight

• a novel two-phase heat transfer device is tested with a parabolic flight campaign. • the performances are tested in different orientations, power and gravity level. • the Capillary Jet Loop working fluid is R1233zde. • microgravity and hypergravity results show dryout instabilities at zero gravity. • Ejector’s By-Pass Ratio is increased in the vertical orientation. In this study, a prototype Capillary Jet Loop Heat Pipe partially filled with R1233zd(E) has been tested to investigate its thermal behaviour on board of parabolic flights where gravity condition changes in the range from 0 to 1.8 g level with a 22 s window of microgravity. The device has been tested both in horizontal and vertical orientations, varying the thermal load provided to the evaporator from 40 W to 180 W. With power higher than 100 W, the device operated in stationary conditions during hypergravity and normal gravity periods. During the microgravity periods, the device has always shown a sharp increase in pressure and temperatures, reducing temporarily the transport capability of the CJL and fast recovery when gravity increases again. Each series of power level results is presented and discussed in terms of temperature, pressure, and temperature difference from the saturation temperature. The thermal power dissipated by the condenser and an estimation of the condensing flow rate have been calculated from the condenser cooling water inlet and outlet to highlight the Capillary Jet Loop operating phenomena. The temperature of the ejector's primary, secondary, and mixing flows (vapour jet pump) are compared for both configurations.

Deployment of vapour chambers for electronic heat dissipation: state-of-the-art

Miniaturization and high performance are the two principal requirements of modern electronics. All commercial applications demand reliable performance at high operational efficiency for prolonged service life. In order to meet all these requirements, the cooling system should be able to handle large heat fluxes packed in compact spaces and take away the heat as soon as possible to avoid overheating of the chip. Phase change heat transfer devices can handle large heat fluxes on account of latent heating. Lately, vapour chambers (VCs) are found to be promising heat spreaders as they are passive two-phase heat transfer devices. They effectively eliminate the hotspots, transferring high heat fluxes with better efficiencies and offer lower thermal resistance compared to the conventional cooling techniques. A detailed review of the latest research and advancements in the field of VCs is presented in this paper, including the effect of different thermo-physical parameters on their performance. Finally, all the major findings are summarized, including the future scope of development in a systematic manner.

Design of a cryogenic two-phase flow visualization system for cryogenic pulsating heat pipe

A cryogenic pulsating heat pipe (PHP), as a novel two-phase heat transfer device, has been studied for cooling superconducting magnets because of its high transfer coefficient, simple configuration, and flexible arrangement. From the results of the room temperature PHP experiments, it was noticed that the flow characteristics and the flow pattern of the PHP are complex, which affect the heat transfer performance of the PHP. In this work, a cryogenic two-phase flow visualization system for cryogenic PHP by using a GM cryocooler is developed to get a better understanding of the oscillating behavior, the distribution of vapor slug and liquid plug, and the flow pattern of the cryogenic working fluid. The details of the design of this system are presented. The performance of the GM cryocooler is verified and the temperature distribution of the cooling system is discussed. The liquid slugs and vapor bubbles were observed during the cool-down period. The visualization experiment will be carried out with the cryogenic PHP prototype installed and operated in a vertical position using nitrogen as a working fluid.

Characterization of Wicking Performance for Open Rectangular Microgrooves under Planar EHD Effects in Two-Phase Heat Transfer Devices

Characterization of Wicking Performance for Open Rectangular Microgrooves under Planar EHD Effects in Two-Phase Heat Transfer Devices

초정밀 온도제어 및 측정기술 동향

In this work, recent advances in temperature control techniques and the resulting contemporary progress in precision thermometry are addressed together with a broad review of traditional temperature control methods. Particular emphases are placed on clarification of the nature of temperature control and its classification, and the relevant technical issues are addressed based on this clarification and classification. Being a thermodynamic quantity having the same dimension as energy, temperature of an object is traditionally controlled by means of the changing rate of energy (Heat) transfer; however, this approach has led to a slow, less stable, and uneven temperature field due to inherent limits caused by finite properties of materials. To overcome this problem, thermodynamic characteristics of two-phase heat transfer devices, such as heat pipes and loop heat pipes, have been extensively employed where high-speed nature of fluid flow was exploited to realize a uniform temperature field, and unique thermodynamic linkage between saturation temperature and pressure was successfully applied to attain a fast, stable, and predictable temperature control of a finite-sized isothermal space. Representative examples and applications are provided in the context of unique features of the introduced contemporary temperature control techniques, which caused significant scientific strides in the related fields.

Development of two-phase flow regime map for thermally stimulated flows using deep learning and image segmentation technique

Accurate prediction of the flow patterns is important for understanding the behavior and operation of thermo-fluidic phenomena in two-phase systems. Considering the importance of thermally induced flows in capillaries and their use in various processes, the thermo-hydrodynamics of two-phase flows in pulsating heat pipes (PHPs) have been studied. In the present research, flow regimes were identified in capillaries where the mass flow rate is difficult to deal with. A deep learning technique has been carried out to classify the flow patterns and extract their attributes in a particular capillary using multi-spectral segmentation. The modified Weber, Froude, and Bond numbers are used by considering the absolute velocities, accelerations, and bubble lengths for classifying the flow regime. Flow regime maps were plotted based on the proposed non-dimensional numbers by analyzing experimentally measured data sets and visual observation for methanol as a working fluid at six different heat levels (18,38,44,50,60, and 70 W). Four different flow regimes have been identified and classified as a bubbly flow, a slug flow, an elongated plug flow (transitional flow), and an annular/semi-annular flow, and a new flow regime map for thermally induced two-phase flows in a PHP is presented. The flow regime map gives quantifiable criteria for the calculation of bubbly, slug, and annular flow patterns. The present study will be useful for researchers analyzing a large number of two-phase flow images and to design two-phase passive heat transfer devices based on the flow regime map.

Startup characteristics of an ammonia loop heat pipe with a rectangular evaporator

Flat-plate loop heat pipe (FLHP) is a passive two-phase heat transfer device. Comparing with traditional LHP with a cylindrical evaporator, it can be directly connected to a flat heat source without the employment of a saddle, which can effectively reduce the system thermal resistance and enhance the temperature uniformity. In this work, a stainless steel-ammonia FLHP was developed, and extensive experiments have been conducted to investigate its startup characteristics with the evaporator in the horizontal and vertical positions. Experimental results show that the FLHP exhibits excellent startup performance. It can successfully start up at a small heat load as low as 2 W with no obvious temperature overshoot. In a wide power range of 5–35 W, the FLHP generally starts up in only one situation, much simpler than the startup of a LHP with a cylindrical evaporator. For this rectangular evaporator, the heat leak from the evaporator to the compensation chamber (CC) becomes very small. As a result, the vapor can easily exit the condenser in most cases in the power range of 5–35 W, leading to a 100% utilization efficiency of the condenser and the resultant satisfactory thermal performance of the FLHP. In addition, the startup performance and the system thermal resistance of the FLHP are insensitive to the evaporator orientation, promising great application potential in future electronics cooling.