Air heat exchangers with long heat pipes: Experiments and predictions

This paper presents measurements and predictions of a heat pipe-equipped heat exchanger with two filling ratios of R134a, 19% and 59%. The length of the heat pipe, or rather thermosyphon, is long (1.5 m) as compared to its diameter (16 mm). The airflow rate varied from 0.4 to 2.0 kg/s. The temperatures at the evaporator side of the heat pipe varied from 40 to 70 °C and at the condenser part from 20 to 50 °C. The measured performance of the heat pipe has been compared with predictions of two pool boiling models and two filmwise condensation models. A good agreement is found. This study demonstrates that a heat pipe equipped heat exchanger is a good alternative for air–air exchangers in process conditions when air–water cooling is impossible, typically in warmer countries.     

Pool boiling of perfluorocarbon mixtures on silicon surfaces

The need for higher pool boiling critical heat flux (CHF) in electronic cooling applications has turned attention to the use of binary mixtures of dielectric liquids. The available literature demonstrates that the addition of a liquid with higher saturation temperature, higher molecular weight, higher viscosity and higher surface tension can lead to significant enhancement of CHF, beyond what can be achieved through changes in pressure, liquid subcooling, and the product of surface effusivity and heater thickness. The current study focuses on extending the available data on mixture CHF enhancement, as well as pool boiling, on polished silicon surfaces to FC-72/FC-40 mixture ratios of 10%, 15%, and 20% of FC-40 by weight, a pressure range of between 1 and 3 atm, and fluid temperature from 22 to 45 °C, leading to high subcooling conditions. It is found that peak heat flux can be increased to as high as 56.8 W/cm² compared to 25.2 W/cm² for pure FC-72 at 3 atm and 22 °C. It is believed that the increase in the mixture latent heat of evaporation and surface tension, accompanying the depletion of the lower boiling point fluid in the wall region plays the major role in enhancing the critical heat flux for binary mixtures.     

Pool boiling heat transfer on horizontal rectangular fin array in saturated FC-72

The flow patterns and pool boiling heat transfer performance of copper rectangular fin array surfaces immersed in saturated FC-72 were experimentally investigated. The effects of the geometry parameters (fin spacing and fin length) on boiling performance were also examined. The test surfaces were manufactured on a copper block with a base area of 10 mm × 10 mm with three fin spacing (0.5 mm, 1.0 mm and 2.0 mm) and four fin lengths (0.5 mm, 1.0 mm, 2.0 mm and 4.0 mm). All experiments were performed in the saturated state at 1 atmospheric condition. A plain surface was used as the reference standard and compared with the finned surfaces. The photographic images showed different boiling flow patterns among the test surfaces at various heat fluxes. The test results indicated that closer and higher fins yielded a greater flow resistance that against the bubble/vapor lift-off in the adjacent fins. Moreover, as the heat flux approached to critical heat flux (CHF), numerous vapor mushrooms periodically appeared and extruded from the perimeter of the fin array, causing dry-out in the center of the fin array. Closer and higher fins provide more heat transfer. The results also showed that overall heat transfer coefficient decayed rapidly as the fin spacing decreased or the fin length increased. The maximum value of CHF on the base area was 9.8 × 10⁵ W m⁻² for the test surface with a 0.5 mm fin spacing and a 4.0 mm fin length, which has a value five times greater than that of the plain surface.     

Operational characteristics of the miniature loop heat pipe with non-condensable gases

The present paper experimentally investigates the effect of non-condensable gases (NCGs) on the thermal performance of the miniature loop heat pipe (mLHP). Copper mLHP with the flat disk shaped evaporator, 30 mm diameter and 10 mm thick, and fin-and-tube type condenser, 50 mm length and 10 mm height, located at a distance of 150 mm was used in the study. The device which was designed for the thermal control of computer microprocessor was capable of transferring maximum heat load of 70 W while maintaining evaporator temperature below 100 °C limit for electronic equipments. Water was used as the heat transfer fluid inside the mLHP. All the tests were conducted with the evaporator and condenser at the same horizontal level. Simple methods were devised to detect and purge the generated NCG out of the loop heat pipe without disassembling the system. Experiments conducted to classify the trends in the NCG production and storage revealed that majority of the gas is generated in the first few thermal runs and is accumulated in the compensation chamber. Sensitivity tests show that overall effect of the NCG is to elevate the steady-state operating temperature of the loop and increase the start-up time required by the evaporator to achieve stable conditions for the given heat load. As an outcomes of the research work, it can be concluded that mLHPs are more tolerable to the NCGs than conventional heat pipes due to the presence of compensation chamber that can accumulate most of the released gas without major performance degradation.     

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.     

Enhanced flow boiling heat transfer of FC-72 on micro-pin-finned surfaces

For the purpose of cooling electronic components with high heat flux efficiently, some experiments were conducted to study the flow boiling heat transfer performance of FC-72 on silicon chips. Micro-pin-fins were fabricated on the chip surface using a dry etching technique to enhance boiling heat transfer. Three different fluid velocities (0.5, 1 and 2 m/s) and three different liquid subcoolings (15, 25 and 35 K) were performed, respectively. A smooth chip (chip S) and four micro-pin-finned chips with the same fin thickness of 30 μm and different fin heights of 60 μm (chip PF30–60) and 120 μm (chip PF30–120), respectively, were tested. All the micro-pin-finned surfaces show a considerable heat transfer enhancement compared to the smooth one, and the critical heat flux increases in the order of chip S, PF30–60 and PF30–120. For a lower ratio of fin height to fin pitch and/or higher fluid velocity, the fluid velocity has a positive effect on the nucleate boiling curves for the micro-pin-finned surfaces. At the velocities lower than 1 m/s, the micro-pin-finned surfaces show a sharp increase in heat flux with increasing wall superheat, and the wall temperature at the critical heat flux (CHF) is less than the upper limit, 85 °C, for the reliable operation of LSI chips. The CHF values for all surfaces increase with fluid velocity and subcooling. The maximum CHF can reach nearly 150 W/cm² for chip PF30–120 at the fluid velocity of 2 m/s and the liquid subcooling of 35 K.     

Saturated flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip

An experiment is carried out here to investigate flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip flush-mounted in the bottom of a horizontal rectangular channel. Besides, three different micro-structures of the chip surface are examined, namely, the smooth, pin-finned 200 and pin-finned 100 surfaces. The pin-finned 200 and 100 surfaces, respectively, contain micro-pin-fins of size 200 μm × 200 μm × 70 μm (width × length × height) and 100 μm × 100 μm × 70 μm. The pitch of the fins is equal to the fin width for both surfaces. The effects of the FC-72 mass flux, imposed heat flux, and surface micro-structures of the silicon chip on the FC-72 saturated flow boiling characteristics are examined in detail. The experimental data show that an increase in the FC-72 mass flux causes a delay in the boiling incipience. However, the flow boiling heat transfer coefficient is not affected by the coolant mass flux. But adding the micro-pin-fin structures to the chip surfaces can effectively enhance the single-phase convection and flow boiling heat transfer. Moreover, the mean bubble departure diameter and active nucleation site density are reduced for a rise in the FC-72 mass flux. A higher coolant mass flux results in a higher mean bubble departure frequency. Furthermore, larger bubble departure diameter, higher bubble departure frequency, and higher active nucleation site density are observed at a higher imposed heat flux. We also note that adding the micro-pin-fins to the chips decrease the bubble departure diameter and increase the bubble departure frequency. However, the departing bubbles are larger for the pin-finned 100 surface than the pin-finned 200 surface but the bubble departure frequency exhibits an opposite trend. Finally, empirical equations to correlate the present data for the FC-72 single-phase liquid convection and saturated flow boiling heat transfer coefficients and for the bubble characteristics are provided.     

Nucleate boiling heat transfer enhancement for water and FC-72 on titanium oxide and silicon oxide surfaces

An experimental study was performed to investigate the nucleate boiling and critical heat flux (CHF) of water and FC-72 dielectric liquid on hydrophilic titanium oxide (TiO₂) nanoparticle modified surface. A 1 cm² copper heater with 1 μm thick TiO₂ coating was utilized in saturated pool boiling tests with water and highly-wetting FC-72, and its performance was compared to that of a smooth surface. Results showed that TiO₂ coated surface increased CHF by 50.4% and 38.2% for water and FC-72, respectively, and therefore indicated that boiling performance enhancement depends on the level of wettability improvement. A silicon oxide (SiO₂) coated surface, exhibiting similar surface topology, was tested to isolate the roughness related enhancement from the overall enhancement. Data confirmed that hydrophilicity of TiO₂ coated surface provides an additional mechanism for boiling enhancement.     

Effects of heater size and working fluids on nucleate boiling heat transfer

The present study is an experimental investigation of nucleate boiling heat transfer mechanism in pool boiling from wire heaters immersed in saturated FC-72 coolant and water. The vapor volume flow rate departing from a wire during nucleate boiling was determined by measuring the volume of bubbles from the wire utilizing the consecutive-photo method. The effects of the wire size on heat transfer mechanism during a nucleate boiling were investigated, varying 25 μm, 75 μm, and 390 μm, by measuring vapor volume flow rate and the frequency of bubbles departing from a wire immersed in saturated FC-72. One wire diameter of 390 μm was selected and tested in saturated water to investigate the fluid effect on the nucleate boiling heat transfer mechanism. Results of the study showed that an increase in nucleate boiling heat transfer coefficients with reductions in wire diameter was related to the decreased latent heat contribution. The latent heat contribution of boiling heat transfer for the water test was found to be higher than that of FC-72. The frequency of departing bubbles was correlated as a function of bubble diameters.     

Experimental study on CPU cooling system of closed‐loop two‐phase thermosyphon

This paper reports on an indirect cooling method of high‐power CPU of notebook computers using a closed‐loop two‐phase thermosyphon with Fluorinert (FC‐72) as the working fluid. The experimental setup consists of an evaporator with an electric heater, a condenser, and flexible tube connecting them. The heater and condenser act as a high‐power CPU and a cooling plate located behind the display of a notebook computer, respectively. The evaporator and the condenser have the outer dimensions of 50mm × 50mm × 20mm and 150mm × 200mm × 20mm, respectively. Four possible boiling surfaces of an evaporator were examined, i.e., a smooth surface (Type A), rough one, ones with smooth plate fins and rough plate fins (Type D). Type D evaporator shows the highest performance, i.e., it reduces the temperature at the evaporator/heater interface by about 18% in comparison with that of the smooth surface evaporator (Type A). Type D evaporator keeps the temperature difference between the evaporator/heater interface and the ambient to be around 55 K at the highest heat input Q = 30W. The effects of the heat input Q, the volumetric amount of Fluorinert liquid F in the thermosyphon, and the evaporator type on the heat transfer characteristics of the cooling system were examined experimentally. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(3): 147–159, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20057     

Efficient spreaders for cooling high-power computer chips

Investigated is the performance of composite spreaders, consisting of a top layer of porous graphite (⩾0.4 mm), for enhanced cooling by nucleate boiling of FC-72 dielectric liquid, and a copper substrate (⩽1.6 mm), for efficient spreading of the dissipated thermal power by the underling 10 × 10 mm or 15 × 15 mm high-power computer chips. The analysis assumes uniform thermal power dissipation by the chips and calculates the square surface area of the spreader, along with the spreading, boiling and total thermal resistances, the maximum chip temperature, and the removed thermal power from the spreader surface by saturation or subcooled nucleate boiling of FC-72 liquid. These performance parameters are determined as functions of the thickness of the copper substrate and the size of the underlying chip. When compared with those of copper and porous graphite spreaders of the same total thickness, 2.0 mm, the performance of the composite spreaders is superior for cooling high-power computer chips. When cooled by nucleate boiling of 30 K subcooled FC-72 liquid, the composite spreader removes 160.3 W and 98.4 W of dissipated thermal power by the underlying 10 × 10 mm and 15 × 15 mm chips, at total thermal resistances of 0.29 and 0.48 °C/W. When the same spreader is cooled by saturation boiling of FC-72, the removed thermal power decreases to 85.6 W and 53.4 W, and the total thermal resistances also decrease to 0.12 and 0.20 °C/W, respectively. Although the calculated surface temperatures of the chips are not uniform, the maximum temperatures are <71 °C and the temperature differential across the chips is <8 °C. For the same cooling condition, the calculated surface area of the copper spreaders, the total thermal resistance, and the maximum chip temperature are much higher, but the removed thermal powers from the surface of spreaders are much lower than with composite spreaders. The calculated surface areas of the porous graphite spreaders are smaller, the thermal powers removed from surface of these spreaders are much lower and both the total thermal resistance and the maximum chip temperature are higher than those with composite spreaders.     

Instability phenomena in a two-phase microchannel thermosyphon

The behavior of a two-phase thermosyphon, consisting of a microchannel evaporator plate and a condenser, is investigated to gain insight into the system limiting instability. A microchannel plate has been fabricated with 56 square channels that have a 1 × 1 mm cross section and a length of 115 mm. Experiments have been conducted for various condenser heights with the heat flux as the control variable. A step increase in heat flux is used to quantify the response of the system, including variations in mass flow rate, temperature, and pressure drop. Results show that small fluctuations about the steady state give rise to the instability for situations with a uniform heat load. A predictive model based on the momentum equation is introduced to estimate the onset of instability, and the threshold heat flux is predicted to within ±10% uncertainty.     

Operating characteristics of a miniature cryogenic loop heat pipe

Aiming at future space applications, a miniature cryogenic loop heat pipe (CLHP) with nitrogen as the working fluid was designed, whose condenser could provide the interface with the cold finger of cryocooler, and its operating characteristics were experimentally investigated in this work. Based on the experimental results, important conclusions below have been drawn: (1) with only 2.5 W applied to the secondary evaporator, the CLHP can realize the supercritical startup, and the larger the heat load applied to the secondary evaporator, the sooner the temperature drop process of the primary evaporator; (2) when the heat load applied to the primary evaporator is no less than 3 W, the primary evaporator can operate independently; whereas when it is smaller than 3 W, the secondary evaporator must be kept in operation to assist the normal operation of the primary evaporator; (3) the CLHP has a heat transport capacity of 12 W × 0.56 m, and its thermal resistance decreases with the increase of the heat load applied to the primary evaporator; (4) the CLHP has the ability to operate with a small heat load applied to the primary evaporator for a long time, and manifests good thermal control performance.     

Subcooled flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip

Experiments are conducted here to investigate subcooled flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip flush-mounted on the bottom of a horizontal rectangular channel. In the experiments the mass flux is varied from 287 to 431 kg/m² s, coolant inlet subcooling from 2.3 to 4.3 °C, and imposed heat flux from 1 to 10 W/cm². Besides, the silicon chips contain three different geometries of micro-structures, namely, the smooth, pin-finned 200 and pin-finned 100 surfaces. The pin-finned 200 and 100 surfaces, respectively, contain micro-pin-fins of size 200 μm × 200 μm × 70 μm (width × length × height) and 100 μm × 100 μm × 70 μm. The measured data show that the subcooled flow boiling heat transfer coefficient is reduced at increasing inlet liquid subcooling but is little affected by the coolant mass flux. Besides, adding the micro-pin-fin structures to the chip surface can effectively raise the single-phase convection and flow boiling heat transfer coefficients. Moreover, the mean bubble departure diameter and active nucleation site density are reduced for rises in the FC-72 mass flux and inlet liquid subcooling. Increasing coolant mass flux or reducing inlet liquid subcooling results in a higher mean bubble departure frequency. Furthermore, larger bubble departure diameter, higher bubble departure frequency, and higher active nucleation site density are observed as the imposed heat flux is increased. Finally, empirical correlations for the present data for the heat transfer and bubble characteristics in the FC-72 subcooled flow boiling are proposed.     

Boiling of water and FC-72 in microchannels enhanced with novel features

A microchannel test section comprised of parallel square microchannels with a 25 × 25 μm and 50 × 50 μm cross section was manufactured. Boiling of perfluorinated dielectric fluid FC-72 and water in microchannels was studied. Troublesome occurrences associated with flow boiling in microchannels were reduced or eliminated with inlet/outlet restrictors, inlet/outlet manifolds and potential nucleation cavities incorporated in the array of microchannels. The gradual reduction of channel cross section in the manifolds ensured a uniform distribution of the working fluid among the microchannels. The flow restrictors provided a higher upstream pressure drop in comparison with the downstream pressure drop which favors vapor flow in the downstream direction and consequentially suppresses the vapor backflow present in flow boiling. The superheat of the microchannel wall necessary for the onset of boiling was decreased significantly with the incorporation of properly sized artificial cavities. Experimental results confirmed the benefits of the etched features, as there was (i) an even working fluid distribution (ii) without dominating backflows of vapor (iii) at a low temperature of the onset of boiling. Bubble growths as well as other events in the microchannels were visualized with a high-speed imaging system which captured images at over 87,000 frames per second. Results exhibit boiling hysteresis dependence of the working fluid and its mass flux through the microchannels. The temperature of the onset of boiling is highly dependent on the working fluid, microchannel size and its roughness.     

Saturated flow boiling of FC-72 in 1 mm diameter tube

Due to the important role of microscale heat transfer, an analysis of the heat transfer coefficient for a 1 mm inner diameter tube, with FC-72 as working fluid, has been performed. The study is aimed to identify the best correlation or model to predict the available experimental database. A comparison of several models and correlations, available in the literature for micro- and macroscale, has been performed, focusing the present preliminary analysis to the saturated boiling conditions. Experimental data have been provided by ENEA through the facility BO.E.MI.A. (BOiling Experiments in MIcrochannel Apparatus), in the pressure range from 3 to 5 bar, with a mass flux from 800 to 1200 kg/m²s and thermal fluxes from 1.6 to 181 KW/m². The best results, in this preliminary analysis of the saturated boiling points, were obtained for microscale correlations of Li and Wu, with more than 91% of data within ± 30% error and a mean absolute percent error (MAPE) of 13.4%. Among the macroscale correlations only the Chen correlation provides good results, but with a lower agreement with the experimental data. Theoretical models are very promising but need further works to find the appropriate parameters valid for the specific fluid. A more detailed analysis including subcooling conditions will be performed in a future work.     

Experimental investigation of cryogenic oscillating heat pipes

A novel cryogenic heat pipe, oscillating heat pipe (OHP), which consists of an 4 × 18.5 cm evaporator, a 6 × 18.5 cm condenser, and 10 cm length of adiabatic section, has been developed and experimental characterization conducted. Experimental results show that the maximum heat transport capability of the OHP reached 380 W with average temperature difference of 49 °C between the evaporator and condenser when the cryogenic OHP was charged with liquid nitrogen at 48% (v/v) and operated in a horizontal direction. The thermal resistance decreased from 0.256 to 0.112 while the heat load increased from 22.5 to 321.8 W. When the OHP was operated at a steady state and an incremental heat load was added to it, the OHP operation changed from a steady state to an unsteady state until a new steady state was reached. This process can be divided into three regions: (I) unsteady state; (II) transient state; and (III) new steady state. In the steady state, the amplitude of temperature change in the evaporator is smaller than that of the condenser while the temperature response keeps the same frequency both in the evaporator and the condenser. The experimental results also showed that the amplitude of temperature difference between the evaporator and the condenser decreased when the heat load increased.     

Visualization and performance measurement of operating mesh-wicked heat pipes

This work presents visualization of the evaporation/boiling process and thermal measurements of operating horizontal transparent heat pipes. The heat pipes consisted of a two-layered copper mesh wick consisting of 100 and/or 200 mesh screens, a glass tube and water as the working fluid. Experimental results indicated that nucleate boiling was prompted for a wick having a fine 200-mesh bottom layer. When the fluid charge approximately equaled the pore volume in the wick, the water–vapor interface receded into more curved menisci with increasing heat load Q. Thus, larger capillary forces and evaporation areas were attained to meet the increasing need of liquid supply and evaporation rate at the water–vapor interface. At Q = 40 and 45 W, the water film became less than 100 μm and the nucleate boiling observed at lower heat loads disappeared. Optimal thermal characteristics with smallest thermal resistances across the evaporator and lowest overall temperature distributions were found for such a wick/charge combination. Under a smaller charge, partial dry-out was observed in the evaporator. Under a larger charge, liquid recession with increasing heat load was limited and bubbles grew and burst violently at high heat loads. The effects of different wicks and fluid charges on the evaporation/boiling characteristics were discussed.     

Performance of LHPs with a novel design evaporator

In order to realize an excellent heat transfer performance of the LHPs, including the fast start-up and high heat transfer capacity, a new connection design between the evaporator envelope and the wick surface without the clearance was proposed. The LHPs with a cylindrical evaporator, 22 mm diameter and 80 mm long, were fabricated with water as the working fluid and an 70% inventory.Copper wicks made of different particle sizes were used in both the start-up and heat transfer capacity tests. It was experimentally observed that the sintered wick with 139 μm diameter particles had the best heat transfer performance. It achieved a start-up time of only 150 s under 30 W heat load, a heat transfer capacity of 500 W under the allowable evaporator temperature of 85 °C, and a low thermal resistance of 0.070–0.165 °C/W. Internal temperature measurements were also conducted to determine the mechanism of the heat leak, to identify the heat pipe effect, and to compare the heat leak with different wicks corresponding to the change of the heat load during the operation     

Influence of carbon nanotube suspension on the thermal performance of a miniature thermosyphon

An experimental study was carried out to understand the heat transfer performance of a miniature thermosyphon using water-based carbon nanotube (CNT) suspensions as the working fluid. The suspensions consisted of deionized water and multi-wall carbon nanotubes with an average diameter of 15 nm and a length range of 5–15 μm. Experiments were performed under three steady operation pressures of 7.4 kPa, 13.2 kPa and 20 kPa, respectively. Effects of the CNT mass concentration and the operation pressure on the average evaporation and condensation heat transfer coefficients, the critical heat flux and the total heat resistance of the thermosyphon were investigated and discussed. Experimental results show that CNT suspensions can apparently improve the thermal performance of the thermosyphon and there is an optimal CNT mass concentration (about 2.0%) to achieve the maximum heat transfer enhancement. The operation pressure has a significant influence on the enhancement of the evaporation heat transfer coefficient, and slight influences on the enhancement of the critical heat flux. The enhanced heat transfer effect is weak at low heat fluxes while it is increased gradually with increasing the heat flux. The present experiment confirms that the thermal performance of a miniature thermosyphon can be strengthened evidently by using CNT suspensions.