High heat flux spray cooling with ammonia: Investigation of enhanced surfaces for CHF

A spray cooling study was conducted to investigate the effect of enhanced surfaces on Critical Heat Flux (CHF). Test surfaces involved micro-scale indentations and protrusions, macro (mm) scale pyramidal pin fins, and multi-scale structured surfaces, combining macro and micro-scale structures, along with a smooth surface that served as reference. Tests were conducted in a closed loop system using a vapor atomized spray nozzle with ammonia as the working fluid. Nominal flow rates were 1.6 ml/cm² s of liquid and 13.8 ml/cm² s of vapor, resulting in a pressure drop of 48 kPa. Results indicated that the multi-scale structured surface helped increase maximum heat flux limit by 18% over the reference smooth surface, to 910 W/cm² at nominal flow rate. During the additional CHF testing at higher flow rates, most heaters experienced failures before reaching CHF at heat fluxes above 950 W/cm². However, some enhanced surfaces can achieve CHF values of up to ≈1100 W/cm² with ≈67% spray cooling efficiency based on liquid usage. The results also shed some light on the current understanding of the spray cooling heat transfer mechanisms. Enhanced surfaces are found to be capable of retaining more liquid compared to a smooth surface, and efficiently spread the liquid film via capillary force within the structures. This important advantage delays the occurrence of dry patches at high heat fluxes, and leads to higher CHF. The present work demonstrated ammonia spray cooling as a unique alternative for challenging thermal management tasks that call for high heat flux removal while maintaining a low device temperature with a compact and efficient cooling scheme.     

Study on flow characteristics and pressure distribution along a heated channel in subcooled flow boiling

In the present work, an experimental investigation is presented which is focused on the pressure drop along the heated as well as the unheated zone of a flow channel. The experiments were performed by using a 16 mm stainless steel ID tube with a 350 mm heated length followed by a 500 mm unheated extension downstream. Pressure drops and pressure distribution were measured along the flow channel, with water, at mass flow velocities of 6342–9513 kg/m² s and at heat fluxes of 3.37–4.1 MW/m². The whole experiments were conducted near atmospheric pressure by keeping the outlet of the flow channel open to the atmospheric pressure. The experimental results show a reduction of the pressure at the outlet of the heated zone at the onset of significant void (OSV) conditions, caused by the increasing of the average velocity at that zone. That reduction of the pressure, at the outlet of the heated zone, decreases as the inlet (and also the outlet) temperature increases, due to the increasing of the length of the two phase flow at the unheated zone. That phenomenon is backed by photographing the flow in the unheated zone.     

Experimental study of saturated flow boiling heat transfer in an array of staggered micro-pin-fins

This study concerns water saturated flow boiling heat transfer in an array of staggered square micro-pin-fins having a 200 × 200 μm² cross-section by a 670 μm height. Three inlet temperatures of 90, 60, and 30 °C, six mass velocities for each inlet temperature, ranging from 183 to 420 kg/m² s, and outlet pressures between 1.03 and 1.08 bar were tested. Heat fluxes ranged from 23.7 to 248.5 W/cm². Heat transfer coefficient was fairly constant at high quality, insensitive to both quality and mass velocity. Heat transfer was enhanced by inlet subcooling at low quality. Possible heat transfer mechanism was discussed.     

Subcooled flow boiling and microbubble emission boiling phenomena in a partially heated microchannel

A simultaneous visualization and measurement study has been carried out to investigate subcooled flow boiling and microbubble emission boiling (MEB) phenomena of deionized water in a partially heated Pyrex glass microchannel, having a hydraulic diameter of 155 μm, which was integrated with a Platinum microheater. Effects of mass flux, inlet water subcooling and surface condition of the microheater on subcooled flow boiling in microchannels are investigated. It is found that MEB occurred at high inlet subcoolings and at high heat fluxes, where vapor bubbles collapsed into microbubbles after contacting with the surrounding highly subcooled liquid. In the fully-developed MEB regime where the entire microheater was covered by MEB, the mass flux, the inlet water subcooling and the heater surface condition have only small effects on the boiling curves. The occurrence of MEB in microchannel can remove a large amount of heat flux, as high as 14.41 MW/m² at a mass flux of 883.8 kg/m² s, with only a moderate rise in wall temperature. Therefore, MEB is a very promising method for cooling of microelectronic chips. Heat transfer in the fully-developed MEB in the microchannel is presented, which is compared with existing subcooled flow boiling heat transfer correlations for macrochannels.     

Flow pattern transition instability during flow boiling in a single microchannel

We studied the unique characteristics of flow boiling in a single microchannel, including the periodic pressure drop, mass flow rate, and temperature fluctuations, in terms of a long time period. Experiments were conducted using a single horizontal microchannel and deionized water to study boiling instabilities at very small mass and heat flow rate conditions. A Polydimethylsiloxane (PDMS) rectangular single microchannel had a hydraulic diameter of 103.5 μm and a length of 40 mm. A series of piecewise serpentine platinum microheaters were fabricated on the inner bottom wall of the rectangular microchannel to supply thermal energy to the test fluid. Real-time flow visualizations of the flow pattern inside the microchannel were performed simultaneously with measurements of the experimental parameters. Tests were performed for mass fluxes of 170 and 360 kg/m² s and heat fluxes of 200–530 kW/m². The test results showed that the heated wall temperature, pressure drop, and mass flux all fluctuated with a long period and large amplitude. These periodic fluctuations exactly matched the transition of two alternating flow patterns inside the microchannel: a bubbly/slug flow and an elongated slug/semi-annular flow. Therefore, the flow pattern transition instability in the single microchannel caused a cyclic behavior of the wall temperature, pressure drop, and mass flux, and this behavior had a very long period (100–200 s) and large amplitude.     

Correlation of critical heat flux and two-phase friction factor for subcooled convective boiling in structured surface microchannels

Critical heat flux (CHF) and pressure drop of subcooled flow boiling are measured for a microchannel heat sink containing 75 parallel 100 μm × 200 μm structured surface channels. The heated surface is made of a Cu metal sheet with/without 2 μm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water +1 vol.% MCNT additive solution, and FC-72 fluids over a mass velocity range of 820–1600 kg/m² s, with inlet temperatures of 15(8.6)°C, 25(13.6)°C, 44(24.6)°C, and 64(36.6)°C for DI water (FC-72), and heat fluxes up to 600 W/cm². The CHF of subcooled flow boiling of the test fluids in the microchannels is measured parametrically. The two-phase pressure drop is also measured. Both CHF and the two-phase friction factor correlation for one-side heating with two other side-structured surface microchannels are proposed and developed in terms of the relevant parameters.     

Assessment of an active-cooling micro-channel heat sink device,using electro-osmotic flow

Non-uniform heat flux generated by microchips causes “hot spots” in very small areas on the microchip surface. These hot spots are generated by the logic blocks in the microchip bay; however, memory blocks generate lower heat flux on contrast. The goal of this research is to design, fabricate, and test an active cooling micro-channel heat sink device that can operate under atmospheric pressure while achieving high-heat dissipation rate with a reduced chip-backside volume, particularly for spot cooling applications. An experimental setup was assembled and electro-osmotic flow (EOF) was used thus eliminating high pressure pumping system. A flow rate of 82 μL/min was achieved at 400 V of applied EOF voltage. An increase in the cooling fluid (buffer) temperature of 9.6 °C, 29.9 °C, 54.3 °C, and 80.1 °C was achieved for 0.4 W, 1.2 W, 2.1 W, and 4 W of heating powers, respectively. The substrate temperature at the middle of the microchannel was below 80.5 °C for all input power values. The maximum increase in the cooling fluid temperature due to the joule heating was 4.5 °C for 400 V of applied EOF voltage. Numerical calculations of temperatures and flow were conducted and the results were compared to experimental data. Nusselt number (Nu) for the 4 W case reached a maximum of 5.48 at the channel entrance and decreased to reach 4.56 for the rest of the channel. Nu number for EOF was about 10% higher when compared to the pressure driven flow. It was found that using a shorter channel length and an EOF voltage in the range of 400–600 V allows application of a heat flux in the order of 10⁴ W/m², applicable to spot cooling. For elevated voltages, the velocity due to EOF increased, leading to an increase in total heat transfer for a fixed duration of time; however, the joule heating also got elevated with increase in voltage.     

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.     

Heat transfer during condensation of moving steam in a narrow channel

The paper presents the results of experimental investigation of heat transfer and hydrodynamics during condensation of moving steam in a narrow channel of square cross-section 2 mm × 2 mm. The channel had a serpentine shape, the channel length was 660 mm. An experimental cell simulated conditions of heat transfer in the condenser of loop heat pipes. The steam velocity at the channel inlet ranged from 13 to 52 m/s, the pressure was 1 atm. The temperature of the cooling water varied from 70 to 95 °C. The annular flow pattern was noted in the whole range of the regime parameters. There was a clear boundary between the condensation zone and the zone occupied by the condensed phase downstream. Temperature has measured along the channel, and the heat-transfer coefficients have been determined. The coefficient values varied from 10,000 to 55,000 W/K m² depending on the steam velocity at the channel inlet and the cooling temperature. The efficiency of the condenser – heat exchanger has been investigated.     

Assessing the influence of four bonding methods on the thermal contact resistance of open-cell aluminum foam

A thermal application of open-cell aluminum foam typically requires it to be bonded on a substrate. The resulting thermal contact resistance is investigated for four bonding methods. This is done by minimizing the difference between the calculated heat transfer via a zeroth order model and experimental data. The bonded metal foam, used to obtain the experimental data, are manufactured in-house. This allows varying pore size, porosity, aluminum alloy, foam height, air mass flow rate, air inlet temperature and bonding method. The latter is found to have an overwhelming impact. The resulting four thermal contact resistances are: 0.7 × 10^?3 m²K/W for brazing, 0.88 × 10^?3 m²K/W for co-casting, 1.25 × 10^?3 m²K/W for a single-epoxy bonding and 1.88 × 10^?3 m²K/W for a press-fit bonding. The uncertainty on these values is 11%.     

Pressure drop studies on two-phase flow in a uniformly heated vertical tube at pressures up to the critical point

Measurements of two-phase flow pressure drop have been made during a phase-change heat transfer process with refrigerant (R-134a) as a working fluid for a wide range of pressures right up to the critical pressure. The experiments were conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length over a heat flux range of 35–80 kW/m², mass flux range of 1200–2000 kg/m² s, exit quality range of 0.19–0.81 and for reduced pressures ranging from 0.24 to 1 with a fixed inlet subcooling of 3 °C. The measurements were compared with the predictions from the homogeneous flow model, a separated flow model using correlations drawn from the literature for void fraction and frictional pressure drop, and finally, using a flow pattern-based predictive method accounting specifically for bubbly, slug and annular flow regimes. It was found that the best results were obtained with the flow pattern-based approach with a mean deviation of ±20% over the entire pressure range.     

Dry-out CHF correlation for R134a flow boiling in a horizontal helically-coiled tube

An experimental study was carried out to investigate the R134a dry-out critical heat flux (CHF) characteristics in a horizontal helically-coiled tube. The test section was heated uniformly by DC high-power source, and its geometrical parameters are the outer diameter of 10 mm, inner diameter of 8.4 mm, coil diameter of 300 mm, helical pitch of 75 mm and valid heated length of 1.89 m. The experimental parameters are the outlet pressures of 0.30–0.95 MPa, mass fluxes of 60–500 kg m^?2 s^?1, inlet qualities of ?0.36–0.35 and heat fluxes of 7.0 × 10³–5.0 × 10⁴ W m^?2. A method based on Agilent BenchLink Data Logger Pro was developed to determine the occurrence of CHF with a total of 68 T-type thermocouples (0.2 mm) set along the tube for accurate temperature measurement. The characteristics of wall temperatures and the parametric effect on dry-out CHF showed that temperature would jump abruptly at the point of CHF, which usually started to form at the front and offside (270° and 90°) of the outlet cross-section. The CHF values decrease nearly linearly with increasing inlet qualities, while they decrease more acutely with increasing critical qualities, especially under larger mass flux conditions. The mass flux has a positive effect on CHF enhancement, but the pressure has negative one. A new dimensionless correlation was developed to estimate dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes under current experimental conditions and compared to calculated results from Bowring and Shah correlations.     

Post-dryout heat transfer to high-pressure water flowing upward in vertical channels with various flow obstacles

Post-dryout heat transfer to high pressure water was investigated experimentally in vertical tubes and annuli containing various flow obstacles. The operational conditions during the experiments were as follows: mass flux from 500 to 1750 kg/m² s, pressure from 5 to 9 MPa, inlet subcooling from 10 to 40 K and heat flux up to 1.5 MW/m². Five different test sections were used in experiments: three annular test sections with inner diameter 12.7 mm and outer diameter 24.3 mm, containing cylindrical and grid flow obstacles in the upper part, and two tubular test sections with inner diameter 24.3 mm with and without pin flow obstacles. The heated length in all test sections was 3650 mm. The wall temperature was measured with 88 thermocouples located along the inner rod and the outer tube surfaces. Due to the presence of flow obstacles, only developing post-dryout heat transfer was observed. Selected post-dryout heat transfer correlations were compared to the experimental data. It has been concluded that all tested correlations predict significantly higher wall temperatures than those obtained in the present experiment. A simple correction function to the Saha model has been suggested which significantly improves the agreement between the correlation and the present data.     

Performance assessment of a direct expansion air conditioner working with R407C as an R22 alternative

This paper presents an experimental investigation of a direct expansion air conditioner working with R407C as an R22 alternative. Experiments are conducted on a vapor compression refrigeration system using air as a secondary fluid through both the evaporator and the condenser. The influences of the evaporator air inlet temperature (20–32 °C), the evaporator air flow rate (250–700 m³/h) and the evaporator air humidity ratio (9 and 14.5 g_wv/kg_a) at the condenser air temperature and volume flow rate of 35 °C and 850 m³/h, respectively on the system performance are investigated. Experimental results revealed that the evaporator air inlet temperature has pronounced effects on the air exit temperatures, pressures of the evaporator and the condenser, cooling capacity, condenser heat load, compressor pressure ratio and the COP of both refrigerants at humidity ratios of 9 and 14.5 g_wv/kg_a. Significant effects of the evaporator air flow rate are also gathered on the preceding parameters at the same values of mentioned-humidity ratios. The best performance, in terms of operating parameters as well as COP, can be accomplished using R22 compared to R407C. The inlet humidity ratio affects dramatically the performance of vapor compression system using R22 and R407C. The raising up humidity ratio from 9 to 14.5 g_wv/kg_a leads to an augmentation in the average cooling capacity by 29.4% and 38.5% and an enhancement in the average COP by 30% and 24.1% for R22 and R407C, respectively.     

Drop formation of swirl-jet nozzles with high viscous solution in vacuum-new absorbent in spray absorption refrigeration

In this paper, the drop formation properties of a lithium bromide salt solution Trane [Trane, Private communication, 1997, [1]], which is utilized in a new concept of spray absorber, is investigated. In the spray absorber of the absorption refrigeration cycles, the feasibility of forming droplets with an optimum diameter of 300 μm, calculated by the drop absorption model, were studied. To achieve above, a single nozzle spray chamber able to attain a low-pressure of 1.23 kPa (0.178 psia) pressure was built. The nozzles experimentally tested were swirl-jet nozzles. The differential pressure across the nozzles was varied from 50 to 200 kPa (7.25–29 psia). The flow rate in the experiment was varied between 0.018 and 0.043 kg/s (2.376–5.676 lb/min). The flow number that define the effective flow of the selected nozzles were 7.6 × 10^?7, 1.5 × 10^?6 and 2.3 × 10^?6 and the viscosity ratio of this disperse/continuous phase flow was 1300. The nozzles tested were able to produce drop sizes having a mean volumetric diameter (MVD) between 375 μm and 425 μm. Comparison of drop absorption model results to conventional absorber results shows a significant improvement in absorption.     

Experimental study on a small Brayton air refrigerator under −120 °C

A small Brayton air refrigerator with high-speed gas bearing turbo-expander and compact plate-fin heat exchanger was designed and fabricated. Performances of the two key components (turbo-expander and heat exchanger) and the refrigerator as a whole were investigated. The bearings used in the refrigerator were compliant foil journal bearings with elastic support, and stability inspection was conducted on them. The results indicate that the refrigerator had a high cooling capability, and that the gas bearings used here achieved good performance in a wide speed region up to 250,000 rpm. While the expander operated at its design speed (220,000 rpm), the refrigerator had a cooling capacity of 1500 W at ?80 °C, and the lowest no-load refrigeration temperature of ?124.5 °C was obtained. Moreover, the influence of nozzle number and velocity ratio on the turbo-expander and the effect of flow rate and cold inlet temperature on the heat exchanger are discussed in detail.     

The characteristics and visualization of critical heat flux of R-134a flowing in a vertical annular geometry with spacer grids

In the present paper, critical heat flux (CHF) experiments of forced convection boiling were performed to investigate the CHF characteristics of a vertical annular channel with one heated rod and four spacer grids for new refrigerant R-134a. The experiments were conducted under outlet pressure of 11.6, 13, 16 and 20 bar, mass fluxes of 100–600 kg/m² s, and inlet temperatures of 25–40 °C. The parametric trend of the CHF data was well consistent with previous understanding in water. The comparison between the present results with effect of the flow obstacle enhancing CHF and water data in similar geometry shows R-134a can be a modeling fluid for simulating water CHF in high pressure and high temperature condition even for annular geometry. The direct observation of flowing bubble behaviors contributes to enhancing our understanding on the effect of flow obstacles for flow boiling heat transfer.     

Experimental investigation and theoretical analysis of the solar adsorption cooling system in a green building

A solar adsorption cooling system was constructed in the green building of Shanghai Institute of Building Science. The system consisted of evacuated tube solar collector arrays of area 150 m², two adsorption chillers with nominal cooling capacity of 8.5 kW for each and a hot water storage tank of 2.5 m³ in volume. A mathematical model of the system was established. According to experimental results under typical weather condition of Shanghai, the average cooling capacity of the system was 15.3 kW during continuous operation for 8 h. The theoretical analysis of the system was verified and found to agree well with the experimental results. The performance analysis showed that solar radiant intensity had a more distinct influence on the performance of solar adsorption cooling system as compared with ambient temperature. It was observed that the cooling capacity increased with the increase of solar collector area, whereas, solar collecting efficiency varied quite contrary. With the increase of water tank volume, cooling capacity decreased, while, the solar collecting efficiency increased. The system performances can be enhanced by increasing the height-to-diameter ratio of water tank. Additionally, it was observed that solar collecting efficiency decreased with the increase of the initial temperature of water in the tank; however, cooling capacity varied on the contrary. Also can be seen is that optimum nondimensional mass flow rate is 0.7 when the specific mass flow rate exceeds 0.012 kg/m² s.     

Investigation of heat transfer and pressure drop of CO2 two-phase flow in a horizontal minichannel

An innovative cooling system based on evaporative CO₂ two-phase flow is under investigation for the tracker detectors upgrade at CERN (European Organization for Nuclear Research). The radiation hardness and the excellent thermodynamic properties emphasize carbon dioxide as a cooling agent in the foreseen minichannels. A circular stainless steel tube in horizontal orientation with an inner diameter of 1.42 mm and a length of 0.3 m has been used as a test section to perform the step-wise scanning of the vapor quality in the entire two-phase region. To characterize the heat transfer and the pressure drop depending on the vapor quality in the tube, measurements have been performed by varying the mass flux from 300 to 600 kg/m² s, the heat flux from 7.5 to 29.8 kW/m² and the saturation temperature from ?40 to 0 °C (reduced pressures from 0.136 to 0.472). Heat transfer coefficients between 4 kW/m² K and 28 kW/m² K and pressure gradients up to 75 kPa/m were registered. The measured data was analyzed corresponding to the dependencies on heat flux, mass flux and saturation temperature. A database has been established containing about 2000 measurement points. The experimental data was compared with common models recently developed by Cheng et al. [1], [2] to cross check their applicability. The overall trends and experimental data were reproduced as predicted by the models before the dryout onset, and deviations have been analyzed. A modified friction factor for the pressure drop model [1] in mist flow has been proposed based on the experimental data.     

Performance of a proposed complete wetting surface counter flow channel type liquid desiccant air dehumidifier

An idea that improves the wettability over the surfaces of a cylindrical dehumidifier channel was proposed and experimentally proved. Fibrous sheets were attached to the inner surfaces of the channel. The capillary effect of fibers sustains the complete wetting of the heat and mass transfer surfaces. The air to be dehumidified and cooled flows upward in the annulus space between the two layers of fibrous sheets, which are saturated with the downward flowing desiccant solution. The permeability of the fibrous sheet was determined experimentally. It was 2.43 × 10^?10 m². The measured solution flow rate due to the capillary suction of the sheets was Γ_in,min = 1.12 kg/h m. The liquid desiccant tested was H₂O/CaCl₂ with salt concentration ratios ranging from 35 to 40%. The measured distribution of the solution flow rate along the circumference of the sheets at the outlet showed 5% deviation from the average flow rate. This is a good indication for the good wettability of walls inside the dehumidifier.Feeding the solution by this mechanism has many advantages over spray feeding. Beside sustaining complete surface wetting, it also prevents channel blockage with solution, which is a main factor in increasing the air pressure drop. About 95% of the air pressure drop is saved in this study by avoiding these problems. A simple theoretical model for the heat and mass transfer processes inside the dehumidifier was developed and experimentally validated. In general, there is good agreement between the predicted and measured data. The developed model was utilized to study the effect of the different parameters on the dehumidifier performance. For a 1 m height dehumidifier with an inlet specific humidity and air temperature of 0.0234 kg_v/kg_a, and 35 °C, respectively, the predicted outlet air specific humidity was 0.0102 kg_v/kg_k and the corresponding outlet air temperature was 27.4 °C. The inlet solution temperature and salt concentration were 25 °C and 40%, respectively.