Boiling flow characteristics in microchannels with very hydrophobic surface to super-hydrophilic surface

Boiling flow process plays a very important role to affect the heat transfer in a microchannel. Different boiling flow modes have been found in the past which leads to different oscillations in temperatures and pressures. However, a very important issue, i.e. the surface wettability effects on the boiling flow modes, has never been discussed. The current experiments fabricated three different microchannels with identical sizes at 105 × 1000 × 30000 μm but at different wettability. The microchannels were made by plasma etching a trench on a silicon wafer. The surface made by the plasma etch process is hydrophilic and has a contact angle of 36° when measured by dipping a water droplet on the surface. The surface can be made hydrophobic by coating a thin layer of low surface energy material and has a contact angle of 103° after the coating. In addition, a vapor–liquid–solid growth process was adopted to grow nanowire arrays on the wafer so that the surface becomes super-hydrophilic with a contact angle close to 0°. Different boiling flow patterns on a surface with different wettability were found, which leads to large difference in temperature oscillations. Periodic oscillation in temperatures was not found in both the hydrophobic and the super-hydrophilic surface. During the experiments, the heat flux imposed on the wall varies from 230 to 354.9 kW/m² and the flow of mass flux into the channel from 50 to 583 kg/m²s. Detailed flow regimes in terms of heat flux versus mass flux are also obtained.     

Experimental investigation of capillary-assisted evaporation on the outside surface of horizontal tubes

Capillary-assisted evaporation is a typical heat transfer method in heat pipes which is characterized by high evaporation coefficient due to extremely thin liquid film. This paper introduces such a micro-scale heat transfer method into normal-scale applications. A series of enhanced heat transfer tubes with circumferential rectangular micro-grooves on the outside surfaces have been experimentally investigated. The aim is to investigate the influence of the tubes’ geometries and operating parameters on the evaporation heat transfer coefficients. In the experiment, the tested tubes are hold horizontally and the bottom surfaces are immersed into a pool of liquid. The heat is added to the thin liquid film inside the micro-grooves through the heating fluid flowing inside the tubes. The factors influencing the capillary-assisted evaporation performance, such as the immersion depth, evaporation pressure, superheating degree, etc. are considered. The experimental results have indicated that there is a positive correlation between the evaporation heat transfer coefficient and evaporation pressure, and negative for the superheating and immersion depth. For water, under the evaporation saturated temperature of 5.0 ± 0.1 °C, the superheating of 4.0 ± 0.1 °C and the dimensionless liquid level of 1/2, the film side evaporation heat transfer coefficients are 3100–3500 W/m² K, which are equivalent to those of the falling film evaporator in LiBr–water absorption machine (2800–4500 W/m² K [Y.Q. Dai, Y.Q. Zheng, LiBr–water Absorption Machine, first ed., Chinese National Defence Industry Press, Beijing, China, 1980.]).     

Experimental study of R-134a evaporation heat transfer in a narrow annular duct

An experiment is carried out here to investigate the evaporation heat transfer and associated evaporating flow pattern for refrigerant R-134a flowing in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm. In the experiment, the effects of the duct gap, refrigerant vapor quality, mass flux and saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. For the duct gap of 2.0 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m² s, imposed heat flux q from 5 to 15 kW/m², vapor quality x_m from 0.05 to 0.95, and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the gap of 1.0 mm, G is varied from 500 to 700 kg/m² s with the other parameters varied in the same ranges as that for δ = 2.0 mm. The experimental data clearly show that the evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher G. Besides, the evaporation heat transfer coefficient also rises substantially at increasing q. Moreover, a significant increase in the evaporation heat transfer coefficient results for a rise in T_sat, but the effects are less pronounced in the narrower duct at a low imposed heat flux and a high refrigerant mass flux. Furthermore, the evaporation heat transfer coefficient increases substantially with the refrigerant mass flux except at low vapor quality. We also note that reducing the duct gap causes a significant increase in h_r. In addition to the heat transfer data, photos of R-134a evaporating flow taken from the duct side show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. Finally, an empirical correlation for the present measured heat transfer coefficient for the R-134a evaporation in the narrow annular ducts is proposed.     

Water droplet evaporation on Cu-based hydrophobic surfaces with nano- and micro-structures

The characteristics of water droplet evaporation on three different hydrophobic surfaces, PCu (Plain Copper, θ = 115°), MSCu (Micro-Structured Copper, θ = 126°) and NSCuO (Nano-Structured Copper Oxide, θ = 159°) with coating of the same SAM (Self-Assembled Monolayer) material, were experimentally investigated. For industrial heat transfer applications, copper material was used as the substrate, and the simple and cost-effective fabrication technique to prepare the superhydrophobic surface, NSCuO, was introduced. Based on the observations, the behavior of droplet evaporation was divided into three stages: Stage I (constant contact area stage), Stage II (constant contact angle stage) and Stage III (mixed stage). When studying the PCu surface, the Stages I, II, and III were observed, consistent with previous reports. For the MSCu surface, Stages I and III appeared without Stage II, and the pinning period of contact line was the longest among the test samples due to the formation of Wenzel state droplet. In the case of the superhydrophobic NSCuO surface, only Stage III occurred, and the contact line moved freely during the entire evaporation time because of the formation of Cassie state droplet. The total evaporation time of the NSCuO was the longest out of all the samples tested. At the last stage of evaporation, the edge of the droplet shrank at a much faster rate in all surfaces. On the other hand, the shrinking velocity of the droplet height drastically increased only on the NSCuO, which was considered as the unique behavior of superhydrophobic surface. In this experiment, it was found that the surface structure determines the motion of the contact line on the surface, which, in turn, strongly influences the characteristics of the droplet evaporation.     

High heat flux phase change on porous carbon nanotube structures

Carbon nanotube (CNT) forests are investigated as porous wick structures for chip-scale heat pipe cooling systems. An analytical model is developed to demonstrate the merits of phase change heat transfer on nanoscale porous structures, compared to that on microscale porous wick. Results indicate that nanoscale porous structures increase the thin-film evaporation surface area by one order of magnitude, which can significantly increase phase change heat transfer efficiency. The pertinent wick structure properties of the CNT forest are experimentally measured. Results show that the CNT forest is highly porous (～95% porosity), and possesses large variations in effective thermal conductivity ranging from 0.8 to 180 W/m K. Effective pore size of the CNT wick structure varies between 50 and 180 nm, which can generate capillary pressure up to two orders of magnitude higher than the microscale wick structure. However, its low permeability, about three to four orders of magnitude lower than the traditional wicks, underscores the necessity of bi-porous CNT wick structures. The bi-porous CNT wick structures are composed of nanoscale porous CNT clusters, separated by microscale (～50 μm wide) passages. Experimental results show a maximum heat flux of 770 W/cm² over a 2 mm × 2 mm heating area. With enhanced thin-film evaporation, heat transfer coefficients are improved by up to 100%, compared to the microscale wick. In contrast, the low CHF ～140 W/cm² over a 10 × 10 mm² heating area is caused by vapor occupation of the microscale pores and the reduction of wick permeability.     

Effects of surfactant additive on flow boiling over a microheater under pulse heating

An experimental investigation has been carried out to study effects of surfactant additive on microscale boiling under pulse heating over a Pt microheater (140 × 100 μm²) fabricated in a trapezoidal microchannel (600 μm in width and 150 μm in depth). Experiments are carried out for six different surfactant concentrations of Triton X-100 ranging from 47 ppm to 2103 ppm, for mass flux in the range from 45 kg/m² s to 225 kg/m² s, pulse width in the range from 50 μs to 2 ms, and heat flux in the range from 3 MW/m² to 65 MW/m². As in existing work on pool boiling under steady heating, it is found that nucleate boiling becomes more vigorous and heat transfer is enhanced greatly with the addition of surfactant with maximum boiling heat transfer occurs at the critical micelle concentration (cmc). Furthermore, these maximum values of boiling heat transfer coefficient increase with decreasing pulse width. When concentration is below cmc, the heat flux needed for nucleation increases with increasing concentration and the nucleation temperature is reduced. When concentration is higher than cmc, the boiling heat transfer coefficient decreases and nucleation temperature is higher than that of pure water.     

On the dynamics and heat transfer of bubble train in micro-channel flow boiling

The dynamics and heat transfer characteristics of flow boiling bubble train moving in a micro channel is studied numerically. The coupled level set and volume of fluid (CLSVOF) is utilized to track interface and a non-equilibrium phase change model is applied to calculate the interface temperature as well as heat flux jump. The working fluid is R134a and the wall material is aluminum. The fluid enters the channel with a constant mass flux (335 kg/m² 1 s), and the boundary wall is heated with constant heat flux (14 kW/m²). The growth of bubbles and the transition of flow regime are compared to an experimental visualization. Moreover, the bubble evaporation rate and wall heat transfer coefficient have been examined, respectively. Local heat transfer is significantly enhanced by evaporation occurring vicinity of interface of the bubbles. The local wall temperature is found to be dependent on the thickness of the liquid film between the bubble train and the wall.     

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.     

Experimental study of evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal small tubes

An experiment is carried out here to investigate the characteristics of the evaporation heat transfer for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m² s, imposed heat flux q from 5 to 15 kW/m², inlet vapor quality x_in from 0.2 to 0.8 and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the 0.83-mm tubes, G is varied from 800 to 1500 kg/m² s with the other parameters varied in the same ranges as those for D_i = 2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. The experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rises in the imposed heat flux, refrigerant mass flux and saturation temperature. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the smaller tubes (D_i = 0.83 mm) may decline at increasing vapor quality when the quality is high, due to the partial dryout of the refrigerant flow in the smaller tubes at these conditions. We also note that under the same x_in, T_sat, G, q and D_i, refrigerant R-407C has a higher h_r when compared with that for R-134a. Finally, an empirical correlation for the R-134a and R-407C evaporation heat transfer coefficients in the small tubes is proposed.     

Boiling enhancement by TiO2 nanoparticle deposition

TiO₂ nanoparticle-coated nickel wires were produced by electrical heating in various nanofluid concentrations ranging from 0.01 to 1 wt.% with various processing heat fluxes from 0 to 1000 kW/m². The experimental results demonstrated up to 82.7% enhancement on critical heat flux (CHF) in condition of coated nickel wire (processed in 1 wt.% with 1000 kW/m²) boiling in pure water. The contact angle measurement revealed that the hydrophilic porous coating formed by vigorous vaporization of TiO₂ nanofluid in nucleate boiling regime enormously modified the wettability of heating surface consequently improving the CHF. Besides, it is evident that the coverage of nanoparticle deposition tended to become more complete as concentration and processing heat flux increased based on SEM and EDS analysis. The nanoparticles dispersed in base fluid exhibited little effect on CHF enhancement and could even hinder the percentage of CHF augmentation from boosting, which demonstrated that one could enhance CHF by using only small amount of nanoparticles just adequate to form surface coatings instead of preparing working fluid with great bulk. However, according to the boiling curves in all cases of coated nickel wires, it is supposed that the nucleate boiling heat transfer coefficient deteriorates as a result of thermal resistance resulted from the occurrence of nanoparticle deposition. In summary, the coated porous structure of nanoparticles leads to enhance CHF and to decrease boiling heat transfer coefficient.     

Visualization and thermal resistance measurement for the sintered mesh-wick evaporator in operating flat-plate heat pipes

This work presents visualization and measurement of the evaporation resistance for operating flat-plate heat pipes with sintered multi-layer copper-mesh wick. A glass plate was adopted as the top wall for visualization. The multi-layer copper-mesh wick was sintered on the copper bottom plate. With different combinations of 100 and 200 mesh screens, the wick thickness ranged from 0.26 mm to 0.8 mm. Uniform heating was applied to the base plate near one end with a heated surface of 1.1 × 1.1 cm². At the other end was a cooling water jacket. At various water charges, the evaporation resistances were measured with evaporation behavior visualized for heat fluxes of 16–100 W/cm². Quiescent surface evaporation without nucleate boiling was observed for all test conditions. With heat flux increased, the water film receded and the evaporation resistance reduced. The minimum evaporation resistances were found when a thin water film was sustained in the bottom mesh layer. With heat flux further increased, partial dryout appeared with dry patches in the bottom mesh holes, first at the upstream end of the heated area and then expanded across the evaporator. The evaporation resistance re-rose in response to the appearance and expansion of partial dryout. When the fine 200 mesh screen was used as the bottom layer, its thinner thickness and stronger capillarity led to smaller minimum evaporation resistances.     

Evaporation heat transfer and pressure drop of HFC-134a in a helically coiled concentric tube-in-tube heat exchanger

The two-phase heat transfer coefficient and pressure drop of HFC-134a during evaporation inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786-m long helically coiled tube with refrigerant flowing in the inner tube and heating water flowing in the annulus. The inner tube is made from copper tubing of 9.52 mm outer diameter and 7.2 mm inner diameter. The heat exchanger is fabricated by bending a straight copper tube into a spiral coil. The diameter of coil is 305 mm. The test run are done at average saturated evaporating temperatures ranging between 10 and 20 °C. The mass fluxes are between 400 and 800 kg m⁻² s⁻¹ and the heat fluxes are between 5 and 10 kW m⁻². The inlet quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The pressure drop across the test section is directly measured by a differential pressure transducer. The effects of heat flux, mass flux and, evaporation temperature on the heat transfer coefficients and pressure drop are also discussed. The results from the present experiment are compared with those obtained from the straight tube reported in the literature. New correlations for the convection heat transfer coefficient and pressure drop are proposed for practical applications.     

Local heat transfer coefficient for pool boiling of R-134a and R-123 on smooth and enhanced tubes

The current paper presents experimental investigation of nucleate pool boiling of R-134a and R-123 on enhanced and smooth tubes. The enhanced tubes used were TBIIHP and TBIILP for R-134a and R-123, respectively. Pool boiling data were taken for smooth and enhanced tubes in a single tube test section. Data were taken at a saturation temperature of 4.44 °C. Each test tube had an outside diameter of 19.05 mm and a length of 1 m. The test section was water heated with an insert in the water passage. The insert allowed measurement of local water temperatures down the length of the test tube. Utilizing this instrumentation, local heat transfer coefficients were determined at five locations along the test tube. The heat flux range was 2.5–157.5 kW/m² for the TBIIHP tube and 3.1–73.2 kW/m² for the TBIILP tube. The resulting heat transfer coefficient range was 4146–23255 W/m². °C and 5331–25950 W/m². °C for both tubes, respectively. For smooth tube testing, the heat flux ranges were 7.3–130.7 kW/m² and 7.5–60.7 kW/m² for R-134a and R-123, respectively; with resulting heat transfer coefficient ranges of 1798.9–11,379 W/m². °C and 535.4–3181.8 W/m². °C. The study provided one of the widest heat flux ranges ever examined for these types of tubes and showed significant structure to the pool boiling curve that had not been traditionally observed. Additionally, this paper presented an investigation of enhanced tubes pool boiling models.     

Heat transfer performance in 3D internally finned heat pipe

An experimental study of heat transfer performance in 3D internally finned steel-water heat pipe was carried out in this project. All the main parameters that can significantly influence the heat transfer performance of heat pipe, such as working temperature, heat flux, inclination angle, working fluid fill ratio (defined by the evaporation volume), have been examined. Within the experimental conditions (working temperature 40 °C–95 °C, heat flux 5.0 kw/m²–40 kw/m², inclination angle 2–90°), the evaporation and condensation heat transfer coefficients in 3D internally finned heat pipe are found to be increased by 50–100% and 100–200%, respectively, as compared to the smooth gravity-assisted heat pipe under the same conditions. Therefore, it is concluded that the special structures of 3D-fins on the inner wall can significantly reduce the internal thermal resistance of heat pipe and then greatly enhance its heat transfer performance.     

Experimental and numerical investigation of droplet evaporation under diesel engine conditions

Evaporation of mono-disperse fuel droplets under high temperature and high pressure conditions is investigated. The time-dependent growth of the boundary layer of the droplets and the influence of neighboring droplets are examined analytically. A transient Nusselt number is calculated from numerical data and compared to the quasi-steady correlations available in literature. The analogy between heat and mass transfer is tested considering transient and quasi-steady calculations for the gas phase up to the critical point for a single droplet. The droplet evaporation in a droplet chain is examined numerically. Experimental investigations are performed to examine the influence of neighboring droplets on the drag coefficients. The results are compared with drag coefficient models for single droplets in a temperature range from T = 293–550 K and gas pressure p = 0.1–2 MPa. The experimental data provide basis for model validation in computational fluid dynamics.     

Convective heat transfer in liquid microchannels with hydrophobic and hydrophilic surfaces

Experiments were performed to study the heat transfer characteristics of channel flows of deionized (DI) water, methanol, 50 wt% DI water/50 wt% methanol mixture, and ethanol solution in asymmetrically (one sided heating) heated rectangular microchannels with an aspect ratio (H/W) of 0.56 and the corresponding hydraulic diameters (D) of 129 μm at 5 ? Re ? 240. Local heat transfer coefficients distribution were recorded with both isothermal (273 K) and isoflux (12.6, 18.1, 32.3, 50.5 mW/mm²) heating. The influences of test liquid mass flow rates, test fluids, heating condition (isothermal vs isoflux), and surface condition (hydrophilic vs hydrophobic) on heat transfer behavior were examined. Thermal entrance length were also found and correlated in terms of the relevant parameters.     

Model for predicting evaporation characteristics of vegetable oils droplets based on their fatty acid composition

In this work, a model for predicting evaporation characteristics (constant of evaporation and evaporation time) of cottonseed oil and diesel fuel has been developed and validated experimentally in the temperature range of 684–917 K under atmospheric pressure.The experimental study is based on the fibre-suspended droplet evaporation technique. The theoretical model for predicting evaporation characteristics is based on the determination of transport properties and thermodynamic properties of different phases of cottonseed oil using the properties of its predominant fatty acids (linoleic, oleic and palmitic). Results show that taking into account convection in the quasi-steady model by the correlation of Ranz and Marshall is enough to give a good prediction of the constant of evaporation of diesel fuel in the studied temperature range. For cottonseed oil, the quasi-steady model gives a good prediction for temperatures from 684 K to 773 K while for temperatures from 773 K to 917 K, it is necessary to take into account the convection and the influence of the heating period of the droplet for a good prediction of the constant of evaporation. For the duration of heating and evaporation time, the model gives a rather good prediction for cottonseed oil for the temperature range from 840 K to 917 K.     

Development and tests of ammonia Miniature Loop Heat Pipes with cylindrical evaporators

Miniature Loop Heat Pipes (MLHPs) are an attractive object for development and investigation as quite a promising means for cooling powerful electronics operating in the temperature range from 50 to 100 °C. The paper generalizes and presents the results of development and tests of 15 different variants of ammonia MLHPs with cylindrical evaporators 5 and 6 mm in diameter, which have an active zone length of 20 mm and are equipped with titanium and nickel wicks. As a result of successive efforts aimed at increasing the MLHPs efficiency, it was possible to achieve values of the heat-transfer coefficient close to 162,000 W/m² °C at a value of the heat flux of about 100 × 10⁴ W/m². A maximum heat flux value of about 135 × 10⁴ W/m² was achieved at the heat-transfer coefficient equal approximately to 75,000 W/m² °C.     

Experimental study on condensation and evaporation flow inside horizontal three dimensional enhanced tubes

Experimental investigations of tube side condensation and evaporation in two 3-D enhanced heat transfer (2EHT) tubes were compared to the performance of a smooth surface copper tube. The equivalent outer diameter of all the tubes was 12.7 mm with an inner diameter of 11.5 mm. Both the inner and outer surfaces of the 2EHT tubes are enhanced by longitudinal grooves with a background pattern made up by an array of dimples/embossments. Experimental runs were performed using R410A as the working fluid, over the quality range of 0.2–0.9. For evaporation, the heat transfer coefficient ratio (compares the heat transfer coefficient of the enhanced tube to that of a smooth tube) of the 2EHT tubes is 1.11–1.43 (with an enhanced surface area ratio of 1.03) for mass flux rate that ranges from 80 to 200 kg/m² s. For condensation, the heat transfer coefficient ratio range is 1.1–1.16 (with an enhanced surface area ratio of 1.03) for mass flux that ranges from 80 to 260 kg/m² s. Frictional pressure drop values for the 2EHT tubes are very similar to each other. Heat transfer enhancement in the 2EHT tubes is mainly due to the dimples and grooves in the inner surface that create an increased surface area and interfacial turbulence; producing higher heat flux from wall to working fluid, flow separation, and secondary flows. A comparison was performed to evaluate the enhancement effect of the 2EHT tubes using a defined performance factor and this indicates that the 2EHT tubes provides a better heat transfer coefficient under evaporation conditions.     

Wall temperature measurements with turbulent flow in heated vertical circular/non-circular channels of supercritical pressure carbon-dioxide

Experimental data are presented which illustrate heat transfer characteristics of the turbulent supercritical flow in vertical circular/non-circular channels. The working fluid was carbon-dioxide operating at a constant pressure of 8 MPa. Experiments were conducted at various conditions with inlet bulk fluid temperatures ranging from 15 to 32 °C, imposed heat fluxes from 3 to 180 kW/m², and mass fluxes from 209 to 1230 kg/m² s. The corresponding Reynolds numbers were within the range of 3 × 10⁴ to 1.4 × 10⁵. Wall temperatures are presented for the three channels with different cross-sectional shapes. These were measured by thermocouples installed on the outer surface of the heating section, and are compared with each other at the same heat flux and mass flux conditions.