. Closed Loop Pulsating Heat Pip

Eng. Closed Loop Pulsating Heat Pipes Part B: Visualization and Semi-empirical Modeling. Data centers are becoming more powerful and more integrated with the continuous development of smart cities, which brings us more technological convenience, but also generates a large amount of waste heat.

The second requires spiraled grooves (much like a rifle barrel) along the inside walls which are not tapered. FIGURE 11. Rotating Heat Pipe. The probability of such an event is high with a combination of a high Etvs number, high heat flux, and comparatively low liquid filling ratio ( 50% or lower). The indicators to measure the operating performance of the PHP usually include the temperature of evaporator, the temperature variation amplitude and frequency, and the thermal resistance. doi:10.1126/science.aba3758, PubMed Abstract | CrossRef Full Text | Google Scholar. This all but eliminates the possibility of damage caused by freezing. In addition, some literatures (Zhang and Faghri, 2008; Shafii et al., 2013) pointed out that the proportion of sensible/latent heat varies with the input heat load. oscillating 4) Engineering applications. For cost reasons, the latter is most often the best choice. The input heat load also has an effect on the flow patterns. Advantages of oscillating heat pipe include the ability to work over longer distances than standard heat pipes as well as very good performance when working against gravity when the condenser is below the evaporator. As Celsia has numerous website pages on this subject (see below), we wont do a deep dive here. Sci., 59. doi:10.2298/tsci200729059s, Shang, F., Yang, Q., Liu, C., Fan, S., and Liu, J. FOM for common working fluids for electronics cooling are given in Figure 3 for reference in the selection of working fluids. 3) Theory and numerical simulation. 205 (Nov.1), 11651188. In general, the working fluid needs to meet the following conditions: 1) Appropriate phase change latent heat. Understanding Operational Regimes of Closed Loop Pulsating Heat Pipes: an Experimental Study. Summary of heat dissipation of high-power chips based on PHPs. (2015), the increase in surface roughness will bring greater frictional resistance to motion, which requires more heat to generate driving force. The lower heat transfer coefficient of steam causes the heat transfer efficiency to drop, and the tube wall temperature rises sharply, reaching the heat transfer limit (i.e., the state of dry-out). The heat transfer effect is better, which can be used for heat dissipation of high-power devices. Investigation of the Evacuation Pressure on the Performance of Pulsating Heat Pipe. 28 (11-12), 13121317. A PHP is a complex heat transfer device with a strong thermo-hydraulic coupling governing its performance. Moreover, for the purpose of achieving the unidirectional flow of working fluids, closed-loop PHPs with check valves are introduced (Han et al., 2016). Xing et al. (2015) showed that methanol as a working fluid is often used under high heat load input conditions, showing a better heat transfer performance than ethanol in a wide range of heat load, while ethanol is more suitable for lower heat input. Appl. A PHP consists of a plain meandering tube of capillary dimensions with many U-turns (Figure 1a). Appl. A similar phenomenon will be observed if the system is quasi-statically cooled to a new equilibrium condition A and B (exaggerated representation for clarity) [6]. No use, distribution or reproduction is permitted which does not comply with these terms. 135, 19. New Attempt of Forced-Air Cooling for High Heat-Flux Applications, in Conference on Thermal & Thermomechanical Phenomena in Electronic Systems. Its operation depends on the ability of the working fluid to achieve high enough vapor pressure when heated to push the liquid condensate back to the evaporator section of the device. In contrast to a conventional heat pipe, there is no additional capillary structure inside the tube. Appl. However, using a wick will lower evaporation resistance and increase the maximum power density. At present, the efficient and green cooling scheme is one of the key researches and development points to ensure the stable and safe operation of power electronic devices and achieve energy saving and consumption reduction. Manage. FIGURE 8. Wang X. H. et al. J. More turns will increase the heat exchange area, enhance the pulsation of the working fluids, and weaken the influence of gravity, which is helpful for startup. (2012) and Tseng et al. However, the liquid cooling system is too complicated, and has the risk of working fluid leakage. doi:10.1080/01457630701677114. Experimental Investigation of the Influence of Surfactant on the Heat Transfer Performance of Pulsating Heat Pipe. Available: http://data.chinabaogao.com/it/2021/052K44K62021.html [Accessed]. First, vapor chambers are not simply flat heat pipes. doi:10.1016/j.proeng.2015.05.018. Received: 07 August 2021; Accepted: 30 September 2021;Published: 20 October 2021. Heat Mass Transfer 85, 2328. J. doi:10.1016/j.applthermaleng.2015.08.106, Wang, J., Xie, J., and Liu, X. Appl. Compared to solid metal fins, this type of fin structure is certainly light weight. 110, 390398. According to the surface energy minimization criterion (Srinivasan et al., 2015), the liquid film will rupture and disappear if the thickness of the liquid film is less than a certain value. Yang et al.

Correlation to Predict Heat Transfer Characteristics of a Closed-End Oscillating Heat Pipe at normal Operating Condition. A theoretical model established by Shafii et al. Yue et al. Azar, K., The History of Power Dissipation, ElectronicsCooling, Vol.6, No.1, January 2000. J. Jung, C., and Kim, S. J. Further, thermosyphons can transport heat tens of meters as gravity is used for the liquid return. International Conference on Advanced Vehicle Thermal Management Technologies aims to bring together leading academic scientists, researchers, and research scholars to exchange and share their experiences and research results on all aspects of Advanced Vehicle Thermal Management Technologies.

Heat-generating devices in the server that is the basic component of a data center mainly include processors, storage units, voltage regulators, chips and power supply equipment. Temperature change of (A) the aluminum fin radiator, and (B) the novel vertical pulsating heat pipe radiator under different input power (Xiahou et al., 2019). (2011). Therefore, this paper mainly discusses and analyzes the closed-loop PHPs. Figure 5. (ence and Technology).Heat Transfer Performance of Flat Plate Pulsating Heat Pipe with Double Sides Rectangular or Triangular Channel, Habibi Khalaj, A., and Halgamuge, S. K. (2017). The image above illustrates a loop heat pipe along with an exploded view of the wick structure inside the rectangular reservoir. Simultaneously, point B is forced to move downwards on the vapor line at the other end. In vertical orientation, slug flow is known to exist even at bigger diameters in devices such as a standard bubble pump, depending on the heat flux. Thus, a self-sustained thermally driven oscillating flow is obtained in a PHP. (2019). Numerical simulation studies show that the CPU temperature is negatively correlated with heating power and axial fan pressure, and positively correlated with cooling air temperature. Hemisphere Publishing Corp.McGraw-Hill Book Co. Energy and the Environment - Proceedings of the International Conference on Energy and the Environment, Conference on Thermal & Thermomechanical Phenomena in Electronic Systems, ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems collocated with the ASME 2005 Heat Transfer Summer Conference, Proceedings of the national Conference on Thermal Fluid Science and Tribo Application TFSTA2016), Asme International Conference on Nanochannels), International Conference on Mechanical Engineering: Proceedings of the 11th International Conference on Mechanical Engineering, INTERNATIONAL CONFERENCE ON MECHANICAL ENGINEERING: Proceedings of the 11th International Conference on Mechanical Engineerin, This article is part of the Research Topic, Brief Introduction of Pulsating Heat Pipe, Experimental Research on Influencing Factors, Application of Pulsating Heat Pipe in Data Center, https://doi.org/10.3389/fenrg.2021.755019, http://data.chinabaogao.com/it/2021/052K44K62021.html. Therm. Masanet, E., Shehabi, A., Lei, N., Smith, S., and Koomey, J. 1, some cavities on the inner surface of the capillary will form new microbubbles in the liquid plugs. Also, one or more flow-direction control check valves may be introduced at suitable locations to augment the performance. (2014). In the 1990s, Japanese scientist Akachi proposed the concept of pulsating heat pipe (PHP), which mainly relies on phase change and irregular oscillating motion to achieve high heat flux transfer (Akachi, 1990). Eng. It is estimated that by 2030, the global data center power consumption will reach 974TWh, accounting for 8% of the total global power consumption (Anders, 2021). doi:10.1007/s002310050097, Fourgeaud, L., Nikolayev, V. S., Ercolani, E., Duplat, J., and Gully, P. (2017). 103, 356364. From the above research and analysis, we can see that the working fluid in the alternating cross-section or gradual cross-section channel will add an additional cyclic power due to the non-uniform structure on the basis of the original cyclic driving force, which increases the internal pressure disturbance and effectively drives the directional flow of the internal working fluid. Based on the introduction of the basic structure, working mechanism and outstanding advantages of pulsating heat pipes, this paper reviews in detail the researches on the factors affecting its performance, so as to evaluate the possibility of using pulsating heat pipes in data centers. This reverse pressure difference and the pressure imbalance in the adjacent pipes make the working fluid oscillate irregularity between the two sections with different phase states and flow patterns. Experimental Investigation on the Effect of the Condenser Length on the thermal Performance of a Micro Pulsating Heat Pipe, in Applied Thermal Engineering Design Processes Equipment Economics. However, too high filling rate will lead to fewer vapor slugs and more liquid plugs, which requires a larger thermal driving force and will cause the thermal resistance to increase. As the surface temperature continues to increase, the vapor pressure also increases. doi:10.1016/j.apenergy.2017.08.037, Han, X., Wang, X., Zheng, H., Xu, X., and Chen, G. (2016). Appl. Heat Mass Transfer 55 (21-22), 61096120. The heat transfer of the PHP is mainly composed of the latent heat transfer caused by the phase change of the thin liquid film (Spinato et al., 2016) and the sensible heat transfer caused by the liquid plugs oscillation. Rittidech, S., Terdtoon, P., Murakami, M., Kamonpet, P., and Jompakdee, W. (2003). 20 (2), 150154. Int. Celsia is a custom heat sink manufacturer using vapor chamber and heat pipe technology. According to the model built by Yin et al. J. Their complex operational behavior, which is not yet fully understood, has raised an ever growing academic interest. Zuo, Z. J., North, M. T., and Lee, R. (1999). Oscillating Heat Pipe Simulation Considering Bubble Generation Part I: Presentation of the Model and Effects of a Bubble Generation. J. Int. Heat Mass Transfer 72 (5), 5065. Verma et al. Int. Eng. doi:10.1007/s11630-007-0056-8, Luis, A., Juan, P., and Marcia, M. (2020). J. As a branch of the heat pipe, the pulsating heat pipe is one of most promising passive cooling techniques among many candidates for its unique advantages such as small size, simple and compact structure, and high heat dissipation efficiency, but its application in data centers just begins, and there are few reports on research and implementation. (A) Thermal resistance and (B) average temperature in the evaporation section with different wind speeds for the PHP CPU cooler (Shang et al., 2021). The experimental study conducted by Yang et al. A Review of Current Status of Free Cooling in Datacenters. With the heat source located on the back, leftward side of the reservoir, the liquid ammonia turns to vapor. Performance of a Vertical Closed Pulsating Heat Pipe with Hydroxylated MWNTs Nanofluid. 126, 10691076. In the condenser, the high-temperature liquid plugs and vapor slugs dissipate heat, resulting in bubbles rupture, volume reduction and pressure reduction. (2009) found PHPs can be started smoothly at any inclination angle when the turn number is 40, but this critical turn number may not be suitable for other PHPs devices. Therefore, the higher the heat flow level, the shorter the waiting time, and the easier it is to trigger the PHPs. Sci. Int. 95 (may), 297303. Through visualization experiments, it is found that the flow pattern of working fluids in the two PHPs with special structure is easier to form and maintain annular flow, and the possibility of directional circulation is greater. If the evaporation section is a hydrophobic surface, As the plug slides over the evaporation end, small droplets with large contact angles form on the surface, which tends to film boiling requiring a higher degree of superheat, and the hydrophobic layer increases the heat transfer resistance, making the heat transfer performance of PHPs worse. At this time, the mass-spring-damping model that provides power for fluid movement disappears. In order to make the vapor slug penetrates the liquid plug, the total force acting on the liquid plug will be overcome by the momentum generated by the slugs, namely: Where 0r02vu2rdr is the momentum generated by the expansion of the bubble, (p2p1)r02 is the pressure difference between the two ends of the liquid plug,2r0(cosrcosa) is the resistance caused by the hysteresis of the contact angle, and 2r0(0L1wdx) refers to the frictional shear force received by the liquid plug movement. They are attractive heat transfer elements, which due to their simple design, cost effectiveness, and excellent thermal performance may find wide applications. J. Therm. doi:10.1016/S1359-4311(03)00168-6, Khandekar, S., Dollinger, N., and Groll, M. (2003b). 83, 484493. A Review on Efficient thermal Management of Air- and Liquid-Cooled Data Centers: From Chip to the Cooling System. (2015a). Among them, active cooling consists of forced air cooling, forced liquid cooling, refrigerating and thermoelectric cooling, etc., and passive cooling includes air or liquid natural convection, and phase change heat transfer (evaporation, condensation, boiling, and phase change materials). This makes it all the more difficult to undertake mathematical modeling using conventional techniques. (2017). Heres a link to an excellent video showing an ANSYS simulation of a closed-loop oscillating heat pipe (from YouTube Channel EasyMechLearn). Chien K.-H. et al. Appl. They can be built in materials, fluids, shapes, and sizes that are not possible with conventional capillary-driven heat pipes or vapor chambers. doi:10.1016/S1359-4311(01)00063-1, Tseng, C.-Y., Wu, H.-M., Wong, S.-C., Yang, K.-S., and Wang, C.-C. (2018). As the names suggest, in a closed loop structure, the tube is joined end-to-end (Figure 1b). At this time, most of the liquid plugs and vapor slugs will frequently pass through the evaporator and the condenser to increase and decrease the temperature respectively, which means that the working fluid is more prone to phase change when flowing through the evaporator and the condenser, and the heat transfer efficiency is improved. On the other hand, under certain operating conditions, flow pattern transition to annular flow may occur. The operating mechanism in the PHPs is relatively complicated, and the degree of understanding of it is still insufficient. But the hydrophobic surface results in higher thermal contact resistance, which hinders the heat transfer at the condensation section. TABLE 3. doi:10.1016/j.enconman.2018.12.027. After the condenser section, the tube narrows as the size required for the liquid is quite a bit smaller than required for the vapor. doi:10.1007/s00231-017-2051-2, Lin, Y.-H., Kang, S.-W., and Chen, H.-L. (2008). J. Therm. Convers. It consists of a porous structure with a planar evaporating end and five condensation ends, and is 98mm 60mm 52mm in size. Heat Mass Transfer 156, 119814. doi:10.1016/j.ijheatmasstransfer.2020.119814. This design makes them a much better heat spreader and perfect for applications where a high degree of isothermality is required. However, according to the theoretical analysis of Ma et al. Technol. The critical research results of many scholars on its influencing factors are summarized in Table 2. doi:10.1016/j.ijheatmasstransfer.2012.06.024, Jo, J., Kim, J., and Kim, S. J. In addition to this, studies on the mixing surface (hydrophilic in the evaporator and hydrophobic in the condenser) have shown that the unidirectional flow of working fluids in PHPs can improve its performance (Hao T. et al., 2014, Hao Tingting et al., 2014; Leu and Wu, 2017). As a general rule, thermosyphons are simply wickless heat pipes although they sometimes include a grooved wick that helps increase the surface area of the internal wall and allow liquid condensate to more easily return to the evaporator. doi:10.1016/j.applthermaleng.2007.10.019, Lin, Z., Wang, S., Zhang, W., Lee, E., Zeng, X., and Tang, Y. Servers are an important component of data centers, but they are also the main power consumers. It also allows optimization of the fluid charge, effectively reducing the required liquid. Karthikeyan, V. K., Ramachandran, K., Pillai, B. C., and Brusly Solomon, A. In order to make PHPs operate safely and efficiently, the surface tension is an important factor for the physical properties of working fluids. Appl.

(2005). Different pipe diameters, roughness, working fluids with different physical parameters and temperature operating ranges will all have an impact on the experimental conclusions. The limit temperature of the chip is generally less than 70C (or at most 85C). Miyazaki, Y. doi:10.1615/jenhheattransf.2011002712, Liu, S., Li, J., Dong, X., and Chen, H. (2007). Therefore, it is considered that a smaller surface tension is better. 9. Appl. doi:10.1016/j.applthermaleng.2016.03.032, Jian, Q., Wu, H., and Ping, C. (2012). Aprianingsih, N., Winarta, A., Ariantara, B., Putra, N., Kusrini, E., Juwono, F. H., et al.

Check out each one to learn more about these advanced cooling techniques. However, it is out of the scope of this review. (2001) was used to simulate the vapor slugs and liquid plugs behavior in 2-turns closed-loop and open-loop PHPs. Flat or 3D plate with bosses, through or blind holes, dielectric substrate, etc. doi:10.1016/j.rser.2015.12.350, Hao, T., Ma, X., Lan, Z., Li, N., Zhao, Y., and Ma, H. (2014b). Heat Transfer Eng. Apart from this, some other means, such as mixing working fluids and self-wetting fluids, arranging the evaporator below the condenser and using gravity reflux, can also improve the heat transfer limit of the PHP and delay the phenomenon of dry-out. During operation, the thermal resistance of PHPs decrease as the heating power increase (Hu and Jia, 2011). Nucleation and denucleation are the two modes that enable PHPs to start. doi:10.1016/j.expthermflusci.2011.04.014, Tong, B. Y., Wong, T. N., and Ooi, K. T. (2001). It can be seen that the development of high-efficiency cooling technology is of great significance, whether it is, to ensure the safe and efficient operation of electronic devices, or to save power in the data center and improve the level of energy utilization. To be sure, the heat exchange effect is best when all the heat in the evaporation section can be dissipated through the condensation section. The heat dissipation means include forced air cooling, indirect liquid cooling (microchannel, heat pipe), direct liquid cooling (pool immersion boiling, jet cooling, spray cooling), refrigerating and thermoelectric cooling, phase change materials, etc. The rack level cooling system can realize the direct cooling of the IT equipment in the rack, and determine the amount of cold air by changing the fan speed and adjusting the location of the server to optimize the flow channel. doi:10.1016/j.ijheatmasstransfer.2012.05.068, Daraghmeh, H. M., and Wang, C.-C. (2017). Effects of Oscillation Amplitudes on Heat Transfer Mechanisms of Pulsating Heat Pipes - ScienceDirect. (2008) found that the average temperature difference and thermal resistance are significantly reduced when the working fluid is Ag/H2O nanofluid. TV Temperature of the spherical bubble (C), rglobe Radius of the spherical bubble (m), Pv Internal pressure of the spherical bubble (Pa), RContact Thermal resistance between the heat source and the evaporator (C/W), RPHP Thermal resistance of the PHP (C/W), RCond Heat conduction resistance of the tube wall material (C/W), RAir Heat dissipation resistance of the air side (C/W), THeatsource Average temperature of the heat source (C), Te Average temperature of the evaporator (C), Tc Average temperature of the condenser (C), u Velocity of liquid plug pulsation (m/s), M Number of liquid plugs in each parallel pipe, Keywords: data centers, pulsating heat pipe, heat dissipation, energy saving, review, Citation: Li C and Li J (2021) Passive Cooling Solutions for High Power Server CPUs with Pulsating Heat Pipe Technology: Review. Experimental Study on the Start up Performance of Flat Plate Pulsating Heat Pipe. doi:10.1016/j.applthermaleng.2016.03.163, Wang, J., Ma, H., and Zhu, Q. The hot end overheating leads to secondary heat dissipation difficulties, the cold end undercooling leads to CPU chip surface condensation affects its performance and life, and the manufacturing cost is too high, which also restricts the large-scale application of thermoelectric refrigeration technology. (2008) showed that the 2mm closed-loop PHP has the best thermal performance when the inclination angle is 90, but the thermal performance will no longer be related to the inclination angle when the pipe diameter is reduced to 1mm. Ahmad, H., and Jung, S. Y. Energ. Experimental Investigation of Oscillating Motions in a Flat Plate Pulsating Heat Pipe. However, the power consumption of the data center is a big problem. The liquid and vapor slug transport results because of the pressure pulsations caused in the system. (2020). This prevents the lower pressure working fluid vapor from reaching some or most of the condenser fin area. The sudden increase in vapor pressure will cause a series of liquid plugs and vapor slugs to move quickly and continuously. Transfer 53 (11), 112. From the perspective of friction, the rough surface is not conducive to the oscillation movement in the PHP. J. Therm.

From the above analysis, we know that there are many factors affecting the thermal and hydraulic performance of PHPs, which are intricate. FIGURE 10. Int. CL: Investigation, formal analysis, writing original draft. 48 (4), 815824. Rahman, M. L., Salsabil, Z., Yasmin, N., Nourin, F. N., and Ali, M. (2015b). doi:10.1134/s0018151x10040140, Ebrahimi, K., Jones, G. F., and Fleischer, A. S. (2014). 21 (18), 18451862. However, according to the authors knowledge, there are few research reports on the application of PHPs in data center thermal management so far. FIGURE 5. J. Then the system will again come to a new equilibrium temperature and corresponding saturation pressure, point A and point B, in Figure 3. Heat Mass. J. Therm. (2018). Heat Transfer 21 (2), 405409. 21, 2001, pp. A PHP numerical model established by Nikolayev (2011) shows that the latent heat transfer accounts for a large proportion, and the time average of the contribution of latent heat is 82% in the evaporator and 71% in the condenser. Appl.

On the contrary, Groll and Khandekar (2003) pointed out that a working fluid with a larger surface tension increases the friction force of the liquid plug movement, thereby increasing the flow pressure drop. 28 (1), 4959. (2015). The factors affecting the heat transfer limit include the working fluid types, filling rate, flow rate, pressure, cross-sectional area, inclination angle, liquid film thickness, and wall temperature. doi:10.1016/j.applthermaleng.2016.08.187. In 2020, the growth rate of data center market in China exceeded 40%, and it had been continuously 30% in the past 3years before 2020. (2020). A Review of Data center Cooling Technology, Operating Conditions and the Corresponding Low-Grade Waste Heat Recovery Opportunities.

23 (6), 707719. Fluid Sci. doi:10.1016/j.expthermflusci.2014.02.007. Therm. Jo et al. (2021). At the same time, a higher filling rate will also increase the number of liquid plugs used for pulsating motion, which will further increase the amount of sensible heat transfer. The results demonstrate that most of the heat transfer (95%) is in the form of sensible heat rather than latent heat of vaporization, and the latter is mainly the cause of the oscillating flow. Start-up, Heat Transfer and Flow Characteristics of Silicon-Based Micro Pulsating Heat Pipes. Our team of scientists and engineers have published many OHP resources. In doing so, there will be some evaporation mass transfer from the liquid until equilibrium is reached again. There are currently five levels used to solve the heat dissipation problem of the data center: room level, plenum level, rack level, server level, and chip level. Heat Mass. w represents the shear friction between the working fluid and the tube wall, which can be expressed as: Where f is the fluid friction coefficient, which is related to the flow patterns: for laminar flow (Re2300), f=64ReD, otherwise, f=0.3164Re0.25. Therm. Heat Mass Transfer 130, 912922. Further heating expands the vapor slugs, pushing the slugs of water toward the condenser, much in the same way a coffee percolator works. Therm. Energ. Shi and Pan (2017) found that when the filling rate was 0.350.7, the performance of the PHP was better, and the optimal filling rate changed with the change of the input heat load. Eng. Rev. In the light of these facts, the PHP critical diameter, as suggested above, requires further investigation to be established as a design rule [6]. Rotating heat pipes rely on rotational force to move the liquid back to the evaporator; two different designs are typical and are both incorporated into the image below. doi:10.1007/s11630-011-0450-0, Jafarmadar, S., Azizinia, N., Razmara, N., and Mobadersani, F. (2016). 9:755019. doi: 10.3389/fenrg.2021.755019. 459-464. 102, 7888.

TABLE 1. Some possible disadvantages include start-up issues and operating performance at low temperatures or low power. Fluid Sci. First created in the early 1990s, oscillating heat pipes are the newest member of the two-phase family. 181 (FEB.), 331341. There may exist an optional adiabatic zone in between. (2012) designed and manufactured two kinds of flat plate PHPs with the structure of double-sided rectangular and triangular channel. Eng. Khandekar, S., Groll, M., Charoensawan, P., and Terdtoon, P., Pulsating Heat Pipes: Thermo-fluidic Characteristics and Comparative Study with Single-Phase Thermosyphon, Proceedings 12th International Heat Transfer Conference, ISBN-2-84299-307-1, Grenoble, France, 2002, pp.

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. Closed Loop Pulsating Heat Pip