Understanding operational regimes of closed loop pulsating heat pipes: an experimental study

Increasing performance of electronic components is resulting in higher heat flux dissipation. Two-phase passive devices are proven solutions for modern microelectronics thermal management. In this context, heat pipe research is being continuously pursued evolving newer solutions to suit present requirements. Pulsating heat pipes (PHPs), a relatively new and emerging technology is one such field of investigation. The operating mechanism of PHP is not well understood and the present state of the art cannot predict required design parameters for a given task. The aim of research work presented in this paper is to better understand these mechanisms through experimental investigations.Experiments were conducted on a PHP made of copper capillary tube of 2-mm inner diameter. Three different working fluids viz. water, ethanol and R-123 were employed. The PHP was tested in vertical (bottom heat mode) and horizontal orientation. The results strongly demonstrate the effect of input heat flux and volumetric filling ratio of the working fluid on the thermal performance of the device. Important insight into the operational regimes of the device has been gained.     

Weakly non linear convection in a shallow horizontal cavity under uniform crossed heat fluxes

A weakly nonlinear stability analysis is developed for the unicellular natural convection pattern in a horizontal fluid layer heated by uniform crossed heat fluxes applied on the horizontal and vertical walls, the remaining walls being adiabatic. The critical Rayleigh number is determined, for different Prandtl numbers and heat flux ratios, from the analytical parallel approximation for the single cell flow. For the range of parameters considered, stable, supercritical bifurcations occur solely for longitudinal disturbances with three velocity components. Depending on the value of Pr and the heat flux ratio, the instability is oscillating and hydrodynamic at small Pr and thermal and steady for larger Pr. Computations of the Landau coefficients for the two interacting modes reveal that the hydrodynamic mode is the only stable mode at the codimension points.     

Experimental research on heat transfer of pulsating heat pipe

Experimental research was conducted to understand heat transfer characteristic of pulsating heat pipe in this paper, and the PHP is made of high quality glass capillary tube. Under different fill ratio, heat transfer rate and many other influence factors, the flow patterns were observed in the start-up, transition and stable stage. The effects of heating position on heat transfer were discussed. The experimental results indicate that no annular flow appears in top heating condition. Under different fall ratios and heat transfer rate, the flow pattern in PHP is transferred from bulk flow to semi-annular flow and annular flow, and the performance of heat transfer is improved for down heating case. The experimental results indicate that the total heat resistant of PHP is increased with fill ratio, and heat transfer rate achieves optimum at filling rate 50%. But for pulsating heat pipe with changing diameters the thermal resistance is higher than that with uniform diameters.     

Instability,heat transfer and flow regime in a two-phase flow thermosyphon loop at different diameter evaporator channel

In this study, the influence of different channel geometries on heat transfer, flow regime and instability of a two-phase thermosyphon loop, is investigated. Instabilities in flow regime and heat transfer, at low and high heat fluxes, are observed. Bubbly flow with nucleate boiling heat transfer mechanism, confined bubbly/slug flow with backflow for small channel height (H) and finally slug/churn flow at high heat fluxes are observed. This study shows that flow and thermal instability increases as channel height (H) decreases and also heat transfer coefficient increases with increasing channel height and heat flux. Bubbly flow characterizes the flow regime at high heat transfer coefficients while confined bubbles, backflow and intermittent boiling are more significant for low channel heights with lower heat transfer coefficient and critical heat flux.     

Experimental investigations on operating characteristics of a closed loop pulsating heat pipe

The operating mechanism of the pulsating heat pipe (PHP) is not well understood and the present technology cannot predict required design parameters for a given task. The aim of research work presented in this paper is to better understand the operation regimes of the PHP through experimental investigations. A series of experiments were conducted on a closed loop PHP with 5 turns made of copper capillary tube of 2 mm in inner diameter. Two different working fluids viz. ethanol and acetone were employed. The operating characteristics were studied for the variation of heat input, filling ratio (FR) and inclination angle of the tested device. The results strongly demonstrate the effect of the filling ratio of the working fluid on the operational stability and heat transfer capability of the device. Important insight into the operational characteristics of PHP has been obtained.     

A forced convection subcooled boiling model for nonuniform axial heat fluxes

The modeling of convective subcooled boiling of water flowing in round tubes subjected to nonuniform axial heat fluxes is described. The effects of different axial heat flux profiles are modeled using a local hypothesis; i.e. flow and thermal development are assumed to occur very rapidly in the subcooled boiling (SCB) flow regime. A computer code has been developed to predict the pressure drop, heat transfer coefficient and wall temperature for nonuniform axial heat fluxes, starting with a well-validated code for uniform axial heat fluxes. The predictions for some common nonuniform axial heat profiles are compared to the uniform heat flux case.     

Effect of tube bundles on nucleate boiling and critical heat flux

In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 312–325, 1998     

Microchannel size effects on local flow boiling heat transfer to a dielectric fluid

Heat transfer with liquid–vapor phase change in microchannels can support very high heat fluxes for use in applications such as the thermal management of high-performance electronics. However, the effects of channel cross-sectional dimensions on the two-phase heat transfer coefficient and pressure drop have not been investigated extensively. In the present work, experiments are conducted to investigate the local flow boiling heat transfer of a dielectric fluid, Fluorinert FC-77, in microchannel heat sinks. Experiments are performed for mass fluxes ranging from 250 to 1600 kg/m² s. Seven different test pieces made from silicon and consisting of parallel microchannels with nominal widths ranging from 100 to 5850 μm, all with a nominal depth of 400 μm, are considered. An array of temperature sensors on the substrate allows for resolution of local temperatures and heat transfer coefficients. The results of this study show that for microchannels of width 400 μm and greater, the heat transfer coefficients corresponding to a fixed wall heat flux as well as the boiling curves are independent of channel size. Also, heat transfer coefficients and boiling curves are independent of mass flux in the nucleate boiling region for a fixed channel size, but are affected by mass flux as convective boiling dominates. A strong dependence of pressure drop on both channel size and mass flux is observed. The experimental results are compared to predictions from a number of existing correlations for both pool boiling and flow boiling heat transfer.     

Boiling heat transfer and dryout phenomenon of CO2 in a horizontal smooth tube

Evaporation heat transfer characteristics of carbon dioxide (CO₂) in a horizontal tube are experimentally investigated. The test tube has an inner diameter of 6.0 mm, a wall thickness of 1.0 mm, and a length of 1.4 m. Experiments are conducted at saturation temperatures of 5 and 10 °C, mass fluxes from 170 to 320 kg/m² s and heat fluxes from 10 to 20 kW/m². Partial dryout of CO₂ occurs at a lower quality as compared to the conventional refrigerants due to a higher bubble growth within the liquid film and a higher liquid droplet entrainment, resulting a rapid decrease of heat transfer coefficients. The effects of mass flux, heat flux, and evaporating temperature are explained by introducing unique properties of CO₂, flow patterns, and dryout phenomenon. In addition, the heat transfer coefficient of CO₂ is on average 47% higher than that of R134a at the same operating conditions. The Gungor and Winterton correlation shows poor prediction of the boiling heat transfer coefficient of CO₂ at low mass flux, while it yields good estimation at high mass flux.     

Crossflow boiling heat transfer in tube bundles

Experimental studies are conducted for two-phase crossflows in horizontal tube bundles at various mass fluxes and local flow qualities. The results are compared with those for a single tube in a channel [4,6] at two-phase crossflow conditions. The difference between the heat transfer performance for a heated tube in an infinite pool and in a heated or non-heated bundle can be explained in terms of the different flow field geometry and thermal environment.     

Measurement of heat flux from steaming ground

The total thermal flux at the surface of ‘steaming ground’ consists of a convective and a conductive component, even in the absence of any visible steam discharge at the surface. The total flux and its convective component can be measured separately and directly using a water-filled ground calorimeter. The conductive component is given by the difference between the two fluxes, but can also be assessed independently using measured near-surface soil parameters and temperature gradients, retaining the thermal conductivity as parameter. The conductivity is controlled, in turn, by the thermal diffusivity and the specific moisture content of the near-surface layer. The observed total flux values range between 0.03 and 2 kW/m² at sites where boiling temperatures occur at depths of about 4 m and <0.1 m, respectively; the convective flux can reach 50% of the total flux at most sites. Analysis of various soil parameters and soil temperature sections points to a ‘heat pipe’ transfer mechanism that maintains a high conductive transfer in a thin near-surface layer where sub-surface steam condensation is enhanced. An empirical power-law function can be used to assess the total heat flux from the boiling point depth at single sites with known soil temperature profiles.     

Transient characteristics of thermal contact conductance between isotropic rough surfaces of metals

Transient measurements of thermal contact conductance are made on the interface between isotropic rough surfaces of metals in air. We present an analytical solution for temperature distribution of the one‐dimensional symmetric system with the condition of time‐dependent temperatures at two points in each body, and thereby interface temperature drops and heat fluxes can be obtained without the condition of heat‐flux continuity at the interface. Contacting surfaces of rod samples (Naval brass, JIS?SK5 carbon tool steel, and JIS?SUS 304 stainless steel) of 25‐mm diameter are uniformly polished using an emery 320 paper. Transient characteristics of both temperatures and heat fluxes at the interface are experimentally determined using the analytical solution. It is revealed through the transient experiment that the thermal contact conductances are not constant at the early stage, but rapidly increase from zero and that the discontinuity of interface heat‐flux is observed by about 20 percent for all metal pairs. For the contact between dissimilar metals, the dependence of thermal contact conductance on the direction of heat flow is not distinguishable. © 2001 Scripta Technica, Heat Trans Asian Res, 30(4): 341–356, 2001     

Modelling of heat transfer between two rollers in dry friction

An analysis of heat transfer between two rollers in dry friction is presented in this paper. The contact is peripheral and is assumed to be imperfect. The heat transfer at the interface is modelled by a thermal contact resistance. The heat flux is generated by dry friction at the interface. The two rollers are cooled by convection. A numerical model has been developed to determine the steady state temperature in rollers. Taking into account the transport phenomenon due to motion, the mesh is correlated with the velocity. The accuracy of the mesh is validated by comparison with an available analytical solution developed for a single roller in rotation. The thermal behaviour is analysed with respect to: (i) the velocity, (ii) the heat convection coefficient, and (iii) the thermal contact resistance. The evolutions of the temperature and the partition coefficient of frictional heat are presented and discussed.     

Transition and flow boiling heat transfer inside a horizontal tube

Experiments on transition and flow boiling heat transfer with refrigerant R114 inside a horizontal tube were performed at bubble flow, critical heat flux and in the transition region between bubble flow and film boiling at mass fluxes between 1200 and 4000 kg/m² s and in the pressure range between 5 and 15 bar. In comparison with pool boiling bubble flow heat transfer depends essentially on the mass flow rates and on the vapor quality. The critical heat flux depends less on the temperature difference than in pool boiling heat transfer and exhibits a maximal and a minimal value as a function of the pressure. The critical heat flux increases with mass flow rate as already shown by Collier. In the region of transition boiling the heat flux over the difference between wall and saturation temperature approaches a horizontal curve. Therefore in this region an evaporator may always be operated under stable conditions and burn out does not occur.     

Experimental analysis of flow and heat transfer in a miniature porous heat sink for high heat flux application

A novel miniature porous heat sink system was presented for dissipating high heat fluxes of electronic device, and its operational principle and characteristics were analyzed. The flow and heat transfer of miniature porous heat sink was experimentally investigated at high heat fluxes. It was observed that the heat load of up to 280 W (heat flux of 140 W/cm²) was removed by the heat sink with the coolant pressure drop of about 34 kPa across the heat sink system and the heater junction temperature of 62.9 °C at the coolant flow rate of 6.2 cm³/s. Nu number of heat sink increased with the increase of Re number, and maximum value of 323 for Nu was achieved at highest Re of 518. The overall heat transfer coefficient of heat sink increased with the increase of coolant flow rate and heat load, and the maximal heat transfer coefficient was 36.8 kW(m² °C)^?1 in the experiment. The minimum value of 0.16 °C/W for the whole thermal resistance of heat sink was achieved at flow rate of 6.2 cm³/s, and increasing coolant flow rate and heat fluxes could lead to the decrease in thermal resistance. The micro heat sink has good performance for electronics cooling at high heat fluxes, and it can improve the reliability and lifetime of electronic device.     

Boiling Heat Transfer Performance in a Spiraling Radial Inflow Microchannel Cold Plate

ABSTRACT

This study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling, and mass fluxes are considered. The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 microns. The channel is heated on one side, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface and the other was a surface coated with zinc oxide nanostructures that are superhydrophilic. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter, as indicated by reduced temperature oscillations during boiling, and achieving higher maximum heat flux without dryout. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm² with an average surface temperature of 134° Celsius. Recommendations for electronics cooling applications are also discussed.     

Local heat transfer distribution and effect of instabilities during flow boiling in a silicon microchannel heat sink

Flow boiling of the perfluorinated dielectric fluid FC-77 in a silicon microchannel heat sink is investigated. The heat sink contains 60 parallel microchannels each of 100 μm width and 389 μm depth. Twenty-five evenly distributed temperature sensors in the substrate yield local heat transfer coefficients. The pressure drop across the channels is also measured. Experiments are conducted at five flow rates through the heat sink in the range of 20–80 ml/min with the inlet subcooling held at 26 K in all the tests. At each flow rate, the uniform heat input to the substrate is increased in steps so that the fluid experiences flow regimes from single-phase liquid flow to the occurrence of critical heat flux (CHF). In the upstream region of the channels, the flow develops from single-phase liquid flow at low heat fluxes to pulsating two-phase flow at high heat fluxes during flow instability that commences at a threshold heat flux in the range of 30.5–62.3 W/cm² depending on the flow rate. In the downstream region, progressive flow patterns from bubbly flow, slug flow, elongated bubbles or annular flow, alternating wispy-annular and churn flow, and wall dryout at highest heat fluxes are observed. As a result, the heat transfer coefficients in the downstream region experience substantial variations over the entire heat flux range, based on which five distinct boiling regimes are identified. In contrast, the heat transfer coefficient midway along the channels remains relatively constant over the heat flux range tested. Due to changes in flow patterns during flow instability, the heat transfer is enhanced both in the downstream region (prior to extended wall dryout) and in the upstream region. A previous study by the authors found no effect of instabilities during flow boiling in a heat sink with larger microchannels (each 300 μm wide and 389 μm deep); it appears therefore that the effect of instabilities on heat transfer is amplified in smaller-sized channels. While CHF increases with increasing flow rate, the pressure drop across the channels has only a minimal dependence on flow rate once boiling is initiated in the microchannels, and varies almost linearly with increasing heat flux.     

Effect of Surface Tension Variation of the Working Fluid on the Performance of a Closed Loop Pulsating Heat Pipe

In this article, the effect of surface tension variation of the working fluid on the thermal performance of a pulsating heat pipe (PHP) is presented. A two-turn closed loop PHP is fabricated with a combination of copper and quartz tubes having 2 mm inner diameter. Three filling ratios are considered in the present study. A common surfactant namely sodium dodecyl sulfate is used to vary the surface tension of the distilled water which is the base working fluid tested in the PHP. Visualization as well as heat transfer studies is performed in the PHP. Thermal resistances of working fluids with different surface tension are estimated for each filling ratio. Further, the effect of surfactant concentration on the hydrodynamics of the PHP is also discussed. Addition of surfactant induces formation of foam after the filling process and in some phase of PHP operation. It changes the flow regime boundaries and lowers the evaporator temperature under specific operating conditions. A flow regime map is also constructed for the present PHP. The working fluid with a lower surface tension gives a minimum evaporator temperature in the vertical orientation and thus, lowest thermal resistance has been obtained. However, surface tension does not influence the performance in the horizontal position appreciably.     

Experimental Study on a Hydrogen Closed Loop Pulsating Heat Pipe with Different Adiabatic Lengths

Pulsating heat pipes (PHPs) are promising heat transfer devices due to their high heat transfer rate, simple construction, and low manufacturing cost. Most previous investigations focused mainly on their performances at room temperature. In order to have a better understanding on their thermal performances in the cryogenic field, a PHP element with liquid hydrogen as the working fluid has been fabricated. In this paper, the effect of adiabatic length (100 and 500 mm) on the thermal performance of the hydrogen PHP was investigated in order to determine whether it is capable of transferring heat over a long distance. Experiments were carried out at the filling ratio of 51% and the heat loads from 0 to 14 W. It can be observed that the two PHPs are able to maintain stable temperature profiles during the operation as the heat load increases. Their thermal resistances are almost constant around 0.3 K/W, but that of the longer PHP is about 30% larger. The results demonstrate the potential advantages of PHPs to effectively transport heat over a long distance.     

High heat flux thermal management through liquid metal driven with electromagnetic induction pump

In this paper, a novel liquid metal-based minichannel heat dissipation method was developed for cooling electric devices with high heat flux. A high-performance electromagnetic induction pump driven by rotating permanent magnets is designed to achieve a pressure head of 160 kPa and a flow rate of 3.24 L/min, which could enable the liquid metal to remove the waste heat quickly. The liquid metal-based minichannel thermal management system was established and tested experimentally to investigate the pumping capacity and cooling performance. The results show that the liquid metal cooling system can dissipate heat flux up to 242 W/cm² with keeping the temperature rise of the heat source below 50°C. It could remarkably enhance the cooling performance by increasing the rotating speed of permanent magnets. Moreover, thermal contact resistance has a critical importance for the heat dissipation capacity. The liquid metal thermal grease is introduced to efficiently reduce the thermal contact resistance (a decrease of about 7.77 × 10⁻³ °C/W). This paper provides a powerful cooling strategy for thermal management of electric devices with large heat power and high heat flux.