Pool Boiling Heat Transfer Experiments Research Articles

Entropy generation and thermodynamic analysis of pool boiling heat transfer on doubly enhanced tubes

To improve the efficiency of the heat exchanger tube, one smooth tube and four doubly enhanced tubes (EX1, EX2, EX3 and EX4) have been investigated for the pool boiling heat transfer experiments in this paper. The indicate that the pool boiling heat transfer coefficient of the doubly enhanced tubes increased visibly with the augmentation of heat flux through the bubble behavior. Heat transfer reinforcement effect of the doubly enhanced tubes are significantly better than that of the smooth tube. Additionally, pool boiling heat transfer coefficient can be further enhanced by raising the saturate temperature. Entropy generation minimization analysis demonstrates that the heat transfer characteristics of like T-shaped tubes (EX1 and EX2) are superior to that of low fin tubes (EX3 and EX4). Particularly, tube EX1 exhibits higher pool boiling heat transfer efficiency. It is observed that a reasonable fin pitch is more advantageous for improving heat transfer characteristics. The utilization of entropy generation minimization analysis provides theoretical support for the design and optimization of doubly enhanced tubes.

Boiling heat transfer study of Micro-Diamond coated surface using underwater explosion shock wave method

Pool boiling heat transfer experiments are performed on micro-diamond particles coated heating surface (DPC heating surface) and no-coated smooth surface on copper substrates. Here DPC heating surface is fabricated with the aid of underwater explosion shock wave. Experiments are conducted on the substrate surface area of 15 × 15 mm2 using distilled water as the working fluid. The substrate is fixed tightly on the heater block, which is embedded with ceramic cartridge heater of 1500 W. The influence of pool boiling heat transfer is experimentally studied over a range of surface roughness values with particles coated heating surface at atmospheric pressure. The obtained results of DPC surfaces are compared with that of the smooth bare copper surface. A remarkable enhancement of the critical heat flux (CHF) at lower wall superheat is observed on the DPC surfaces. The DPC surface is found to be effective in increasing the nucleate boiling and CHF

Effect of thermal contact resistance on the CHF and HTC for pool boiling heat transfer

In recent years, there has been increasing interest in the demand for heat dissipation of electronic devices. Constructing micro/nanostructures and using nanofluid for the pool boiling heat transfer have proved to be a notoriously effective method to increase the critical heat flux (CHF) as well as the heat transfer coefficient (HTC). When the tested samples with micro/nanostructures are soldered to the upper of the heated copper block, the CHF boiling curves of the tested samples always shift right, which may be caused by the thermal contact resistance (TCR) from the soldering interface. However, it is rarely for the research to be reported the effect of the thermal contact resistance on the pool boiling heat transfer performance. This paper highlights the effect of the TCR on the CHF and HTC for pool boiling heat transfer by comparing the boiling curve of the non-soldering, the soldering, and the Rohsenow curve. The relationship between the difference in temperature with increased heat flux is established by measuring the TCR of the soldering interface, and the relationship at the different soldering interface morphology has been explored. The results show that the CHF boiling curve with soldering shift right 106–144 % than that of the non-soldering, and the HTC decreases by 45.6–63.5 %. The shifted boiling curve could be corrected by the relationship between the TCR with increased heat flux. However, the boiling curve cannot be corrected by the relationship when the soldering interface has voids. This work demonstrates that the deviation between the boiling curve of soldering with the boiling curve of non-soldering is universal for engineering applications by comparing similar studies. Regardless of whether soldering is used in pool boiling heat transfer experiments, the Rohsenow curve is still an important reference, but the effect of TCR must be considered in the pool boiling curve of soldering.

The investigation of heat transfer enhancement by using different mixture conditions of graphene nanofluids on a downward facing surface

Abstract In this study, graphene nanofluids were used to explore the effect of various concentrations on boiling heat transfer of downward-facing heating. Five concentrations of graphene nanofluids were prepared for pool boiling heat transfer experiments. The experimental results show that when the mass concentration is 10 mg/L, the maximum enhancement of the CHF is up to 76.1%. In order to explore the mechanism of graphene nanofluid enhancing boiling heat transfer, after the experiment, the wettability and roughness of the heating surface were measured and the heating surface was characterized by a scanning electron microscope (SEM) and electronic differential system (EDS). The results show that the wettability is enhanced and the surface roughness is reduced. In addition, boiling curves (the curves of heat flux with surface superheat) and the curves of heat transfer coefficient with heat flux at different concentrations have also been observed to further explore the mechanism of enhanced heat transfer.

Pool boiling heat transfer enhancement of R134a, R32, and R600a using reentrant cavity surfaces

ABSTRACT The nucleated pool boiling heat transfer experiments for plain and five different reentrant cavity copper tube surfaces in saturated R134a, R32, and R600a have been carried out. The experiments are carried for heat fluxes ranging from 6.92 to 51.71 kW/m2 at saturation temperatures of 7°C and 10°C. A significant increase in the boiling heat transfer coefficient has been observed for all tube surfaces with the reentrant cavity. The nucleate boiling heat transfer coefficient for R134a, R32, and R600a is 1.2–2.8 times more than that for plain tube surfaces. A correlation for reentrant cavity surfaces has also been developed.

Heat Exchanging Grid Structures Based on Laser-Based Powder Bed Fusion: Formation Process and Boiling Heat Transfer Performance

Microchannel structures possess high efficiency and high boiling heat transfer coefficient of two-phase flow. In particular, the grid structure has the advantages of a simple pattern, large load capacity, and good surface adaptability. Employing the laser-based powder bed fusion (L-PBF) manufacturing technology, a new method of forming heat transfer grids with a controllable structure is proposed in this study. The formation principle, process, and the reasons for improvements in the boiling heat transfer performance were investigated with stainless steel materials. Laser scanning with varying scan spacings was used to prepare multiple structures with different grid widths and wall heights. On this basis, the porosity and pore morphology of the grid structures were analyzed, followed by pool boiling heat transfer experiments. The results revealed that the grid structure significantly affected the nucleate boiling behavior and increased the critical heat flux (CHF). It was found that the 0.5 mm sample exhibited optimum critical heat transfer performance, with an improvement of 10–27% compared to those of the other four samples (minimum of 63.3 W·cm−2 and maximum of 93.9 W·cm−2). In addition, for samples with a grid width greater than 0.5 mm, the boiling slightly decreased by <5%. When the grid width was further increased, the flow resistance effect and the bubble synapse generation effect tended to converge. In these cases, boiling heat transfer only occurred in a single phase along the direction of the medium wall thickness, thus failing to achieve two-phase heat transfer through bubble growth and collapse.

Experimental study of nucleate pool boiling heat transfer on microporous structured by chemical etching method

Cooling of high heat flux devices and electronics devices with a high heat dissipation system uses Boiling Heat Transfer (BHT). In this paper, a pool boiling heat transfer experiments conducted on saturated distilled water for a bare and microstructured copper surface prepared through chemical etching at atmospheric pressure. A two-step chemical etching process is used for the fabrication of copper microstructure. In the first step, HNO3 and CTAB are used, and in the second step, NaOH and (NH4)2S2O8 are used. This practice comes up with simple management of various surface properties that influence critical heat flux (CHF) and boiling heat transfer coefficient (BHTC) like porosity, roughness, contact angle, etc. These properties are examined with the help of required equipment and are found better. The result of the experiment shows a significant enhancement for BHTC. The maximum BHTC is 82.7 kW/m2K, which is a 60 % increment compared to the bare copper surface. The enhancement of BHTC is due to the microstructure formed on the copper surface through chemical etching. Because of this microstructure, surface properties like roughness, surface wettability are improved in favor of boiling heat transfer performance. But due to the hydrophobic nature, chemically etched surfaces show less CHF than the bare copper surface. For repeatability performance, the surfaces' heat transfer coefficient shows a 4.8 % deviation after a four times repeated test.

Pool boiling enhancement and a method of bubble diameter determination on surfaces with deep minichannels

Pool boiling heat transfer experiments were conducted with ethanol, Novec-649 and FC-72 at atmospheric pressure on surfaces with deep minichannels. The minichannels 5.5, 6.0 and 10 mm deep and 0.5–1.2 mm wide were uniformly spaced with a pitch of 2.0 mm on the base surfaces. Heat transfer coefficients obtained with ethanol were similar for all surfaces tested. Novec-649 and FC-72 produced the highest coefficients at heat fluxes above 100 kW/m2 in 1 mm wide minichannels. All the working fluids provided a significant increase in the maximum (critical) heat flux. The images were recorded using high speed imaging techniques across the entire surface of the samples. Boiling visualization aimed to study vapor bubble diameter changes during the growth period and measure the bubble departure diameter.The bubble diameter at departure was also determined analytically according to force balance static and dynamic variants. The dynamic variant provided a high consistency between the theoretical and experimental data for bubble departure diameter with FC-72 and Novec-649.

Pool boiling heat transfer and bubble behavior on the treelike networks with wedge-shaped channels

This paper designs the treelike networks with the wedge-shaped channel from the idea of bionics. Pool boiling heat transfer experiments of FC-72 under different liquid subcoolings were conducted on the networks with different branch levels. The bubble phenomenon was recorded to reveal the boiling mechanism. The bubble pumpless movement towards the diverging direction in the wedge-shaped channel under the action of capillary force was observed. Under the saturated condition, the surfaces are easier to be completely covered by bubbles and the effect of treelike networks is hindered, which results in similar heat transfer performance. With the increase of liquid subcoolings, the bubble fraction decreases and the effect of the branch level comes into effect. On the surface with high branch level, the bubbles are transported immediately at small radius by the narrow wedge-shaped channel. The moving bubbles disturb the fluid and coalesce with other bubbles, which increases the heat transfer coefficient. There are also more liquid supply pathways between small bubbles, which increases the critical heat flux. The importance of this research is to provide a solution to the difficulty of bubbles vertical departure caused by the lack of buoyancy under microgravity.

Effect of hydrophobic inclined patterns on pool boiling performance of cylindrical copper surfaces

In this study, a screen printing technique was used to fabricate heterogeneous wettable structures on a copper surface. The pool boiling heat transfer experiments were conducted on a hollowed copper cylinder of 25 mm outer diameter and 40 mm in length using deionized water as the working fluid. The effect of heterogeneous wettable surfaces with different inclination angles of the line patterns on the boiling heat transfer performance and bubble dynamics was investigated. Four different inclination angles of interlaced line patterns were evaluated: 0 ° , 30 ° , 60 ° , and 90°. The printed lines were parallel to the axial line of the cylinder for an inclination angle of 0°. The highest heat transfer coefficient enhancements attained for the four angles were 89.27%, 104.59%, 103.99%, and 71.65%, respectively. Toward applications in microelectronics and heat exchangers, the effectiveness of interlaced patterns in a higher heat flux regime, i.e., above 80 kW/m2, is also demonstrated. The surface with θ = 30° pattern lines produced the best heat transfer performance due to the proper distance in the buoyancy direction, causing better bubble coalescence behavior and a better rewetting phenomenon.

Experimental and Mechanism Investigation on Boiling Heat Transfer Characteristics of Alumina/Water Nanofluid on a Cylindrical Tube

Nucleate pool boiling heat transfer experiments have been conducted to nanofluids on a horizontal cylinder tube under atmospheric pressure. The nanofluids are prepared by dispersing Al2O3 nanoparticles into distilled water at concentrations of 0.001, 0.01, 0.1, 1 and 2[Formula: see text]wt.% with or without sodium, 4-dodecylbenzenesulfonate (SDBS). The experimental results showed that: nanofluids at lower concentrations (0.001[Formula: see text]wt.% to 1[Formula: see text]wt.%) can obviously enhance the pool boiling heat transfer performance, but signs of deterioration can be observed at higher concentration (2[Formula: see text]wt.%). The presence of SDBS can obviously enhance the pool boiling heat transfer performance, and with the presence of SDBS, a maximum enhancement ratio of BHTC of 69.88%, and a maximum decrease ratio of super heat of 41.12% can be found in Group NS5 and NS4, respectively. The tube diameter and wall thickness of heating surface are the influential factors for boiling heat transfer coefficient. Besides, we find that Rohsenow formula failed to predict the characteristics of nanofluids. The mechanism study shows that: the decrease of surface tension, which leads to the decrease of bubble departure diameter, and the presence of agglomerates in nanofluids are the reasons for the enhanced pool boiling heat transfer performance. At higher concentration, particle deposition will lead to the decrease of distribution density of the vaporization core, and as a result of that, the boiling heat transfer performance will deteriorate.

Combined effects of liquid wicking and hydrodynamic instability on pool boiling critical heat flux by two-tier copper structures of nanowires and microgrooves

Two-tier structures of Copper Nanowires (CuNWs) and microgrooves have been fabricated on copper substrates. Pool boiling heat transfer experiments have been conducted to study the combined effects of wicking by CuNWs and modulated wavelength by microgroove pitches on pool boiling Critical Heat Flux (CHF). First, 70 nm diameter CuNWs with three different heights of 5 µm, 15 µm, and 25 µm have been synthesized on plain copper surfaces via in-house made Anodic Aluminum Oxide (AAO) templates. The wicking effect of CuNWs at different heights on pool boiling CHF has been measured. Second, microgrooves of 262 µm wide and 518 µm deep at three different mesh pitches of 0.5 mm, 1.0 mm, and 4.0 mm have been fabricated to study the modulated wavelength effect on pool boiling CHF. Last, 25 µm height CuNWs have been integrated with various microgroove meshes to create two-tier structures. The highest pool boiling CHFs of each type of structures are achieved on CuNWs at 25 µm height, microgrooves at 0.5 mm mesh pitch, and two-tier structures of CuNWs at 25 µm height and microgrooves at 0.5 mm mesh pitch are 189.3 W/cm2, 236.4 W/cm2, and 246.3 W/cm2, respectively. The CHF enhancements of these structures over that on a plain surface are 68.6%, 110.5%, and 119.3%, respectively. The experimental results have demonstrated that both wicking effect and modulated wavelength effect contribute to pool boiling CHF enhancement on the two-tier structures, and the collective effect depends on the combination between CuNWs height and microgroove mesh pitch. An existing theoretical model of pool boiling CHF has been extended to include liquid wicking effect and contact angle effect on CuNWs surfaces, and a new correlation for pool boiling CHF on the two-tier structures has been developed to include contributions from both liquid wicking by CuNWs and modulated wavelength by microgrooves.

Effects of Femtosecond Laser Surface Processed Nanoparticle Layers on Pool Boiling Heat Transfer Performance.

An experimental investigation of the effects of layers of nanoparticles formed during femtosecond laser surface processing (FLSP) on pool boiling heat transfer performance has been conducted. Five different stainless steel 304 samples with slightly different surface features were fabricated through FLSP, and pool boiling heat transfer experiments were carried out to study the heat transfer characteristics of each surface. The experiments showed that the layer(s) of nanoparticles developed during the FLSP processes, which overlay FLSP self-organized microstructures, can either improve or degrade boiling heat transfer coefficients (HTC) depending on the overall thickness of the layer(s). This nanoparticle layer thickness is an indirect result of the type of microstructure created. The HTCs were found to decrease with increasing nanoparticle layer thickness. This trend has been attributed to added thermal resistance. Using a focused ion beam milling process and transmission electron microscopy (TEM), the physical and chemical properties of the nanoparticle layers were characterized and used to explain the observed heat transfer results. Results suggest that there is an optimal nanoparticle layer thickness and material composition such that both the HTCs and critical heat flux (CHF) are enhanced.

Effect of heterogeneous wettable structures on pool boiling performance of cylindrical copper surfaces

In this work, pool boiling heat transfer experiments were conducted to investigate the effect of the heterogeneous wettable surfaces. Three different types of heterogeneous wettable surfaces were produced by printing the polymethyl methacrylate (PMMA) polymer on the plain copper and the hydrophobic polymer on the plain copper and superhydrophilic surface. The heat transfer experiments were conducted on a hollow copper cylinder of 25mm outer diameter and 40mm long, with de-ionized (DI) water as the working fluid. The effect of heterogeneous wettability on the boiling bubble dynamics, heat flux, surface temperatures and the average heat transfer coefficient (HTC) were investigated at the saturated conditions. An appreciable enhancement of 98.5% in the HTC was observed on the hydrophobic polymer printed plain copper sample as compared to the plain copper, at a lower heat flux of 15kW/m2.

Pool-Boiling Heat-Transfer Enhancement on Cylindrical Surfaces with Hybrid Wettable Patterns

In this study, pool-boiling heat-transfer experiments were performed to investigate the effect of the number of interlines and the orientation of the hybrid wettable pattern. Hybrid wettable patterns were produced by coating superhydrophilic SiO2 on a masked, hydrophobic, cylindrical copper surface. Using de-ionized (DI) water as the working fluid, pool-boiling heat-transfer studies were conducted on the different surface-treated copper cylinders of a 25-mm diameter and a 40-mm length. The experimental results showed that the number of interlines and the orientation of the hybrid wettable pattern influenced the wall superheat and the HTC. By increasing the number of interlines, the HTC was enhanced when compared to the plain surface. Images obtained from the charge-coupled device (CCD) camera indicated that more bubbles formed on the interlines as compared to other parts. The hybrid wettable pattern with the lowermost section being hydrophobic gave the best heat-transfer coefficient (HTC). The experimental results indicated that the bubble dynamics of the surface is an important factor that determines the nucleate boiling.

Effects of layer-by-layer assembled PEI/MWCNT surfaces on enhanced pool boiling critical heat flux

Layer-by-layer (LbL) assembly is a versatile deposition technique for the fabrication of nanostructured thin films on a substrate. In this study, multilayers of poly(ethylenimine) (PEI) and multi-walled carbon nanotubes (MWCNTs) were deposited onto a stainless steel surface. Since a substrate experiences notable changes in surface characteristics and structures during the LbL deposition process, resulting surface characteristics were thoroughly investigated. It was found that randomly oriented porous structures were developed by the fibrous wires of MWCNTs. As a result, surface roughness increased by 20–30%, and surface wettability increased by 70% for all samples. Both roughness and wettability were improved as the bilayer number increased. After the surface modification process, pool boiling heat transfer experiments were conducted to investigate the thermal margin achievable with the LbL deposition. Critical heat flux (CHF) enhancement was obtained as the number of bilayers increased, with the maximum enhancement evaluated at 94%. The enhancement was mainly attributed to the improved surface characteristics due to the formation of porous structures in the PEI/MWCNT layer. The comparison works with newly proposed CHF correlations supported the wicking effect on the CHF enhancements for the LbL-assembled test samples. Image processing results indicated a desirable structure for improved capillary action, which could affect further wetting to dry spots. High-speed bubble departure images were obtained from which a few msec reduction in rewetting intervals was observed for all LbL-assembled heaters.

Secondary pool boiling effects.

A pool boiling phenomenon referred to as secondary boiling effects is discussed. Based on the experimental trends, a mechanism is proposed that identifies the parameters that lead to this phenomenon. Secondary boiling effects refer to a distinct decrease in the wall superheat temperature near the critical heat flux due to a significant increase in the heat transfer coefficient. Recent pool boiling heat transfer experiments using femtosecond laser processed Inconel, stainless steel, and copper multiscale surfaces consistently displayed secondary boiling effects, which were found to be a result of both temperature drop along the microstructures and nucleation characteristic length scales. The temperature drop is a function of microstructure height and thermal conductivity. An increased microstructure height and a decreased thermal conductivity result in a significant temperature drop along the microstructures. This temperature drop becomes more pronounced at higher heat fluxes and along with the right nucleation characteristic length scales results in a change of the boiling dynamics. Nucleation spreads from the bottom of the microstructure valleys to the top of the microstructures, resulting in a decreased surface superheat with an increasing heat flux. This decrease in the wall superheat at higher heat fluxes is reflected by a "hook back" of the traditional boiling curve and is thus referred to as secondary boiling effects. In addition, a boiling hysteresis during increasing and decreasing heat flux develops due to the secondary boiling effects. This hysteresis further validates the existence of secondary boiling effects.

Pool boiling heat transfer enhancement using vertically aligned carbon nanotube coatings on a copper substrate

In the present study, pool boiling heat transfer experiments are performed on a bare copper surface, a sandblasted copper surface, and carbon nanotube coated copper substrates. Experiments were conducted on the substrate surface area of 14 × 14 mm2 using de-mineralized water as the working fluid. The substrate is fixed tightly on the heater block, which is embedded with six cartridge heaters of total capacity 1500 W. Heat fluxes were measured using J-type thermocouples, which were aligned vertically along the heater block and also 1 mm from the top of the substrate. The pool boiling heat transfer experiments were conducted on bare, sandblasted and vertically aligned CNT coated copper substrates. The obtained results of sandblasted and CNT coated surfaces were compared with that of the bare copper surface. A remarkable enhancement of 38% in the CHF was observed on the CNT coated surface. The CNT coated surface was found to be effective in increasing the nucleate boiling and the CHF.

Pool boiling heat transfer on uniform and non-uniform porous coating surfaces

Pool boiling heat transfer experiments were performed using acetone as the working fluid at atmospheric pressure on seven heater surfaces, which can be divided into three types. The first, second and third types refer to the plain surface with poor heat transfer, the open channel surface and uniform porous coating surface with moderate heat transfer enhancement, and the 2-D/3-D porous coating surfaces with significant heat transfer enhancement. The open channel surface and uniform porous coating surface had intercrossed boiling curves, with better performance for the uniform porous coating surface due to the enhanced nucleation heat transfer at small or moderate wall superheats, and better performance for the open channel surface at large wall superheats due to the vapor channel provided. The 2-D porous coating surface with the pitch width of 1.6mm yields the best heat transfer performance and the largest critical heat fluxes among the three 2-D surfaces. The 3-D porous coating surface (TS#7) significantly increases the critical heat flux, which can be 3.7 times of that for the plain surface. Boiling curves were shifted to smaller wall superheat region and critical heat fluxes were increased with increases in liquid subcoolings. Two distinct bubble patterns existed for 2-D/3-D porous coating surfaces: continuously-isolated-bubble-escape pattern at small or moderate heat fluxes, and periodically-large-bubble-escape pattern at large heat fluxes. The hollow well at the heater center and/or the porous stacks exposed in liquid are acted as the liquid supply duct at high heat fluxes. This explains the reason that the porous coating surfaces with periodically varied particle layer thicknesses had larger critical heat fluxes than other heater surfaces.

B102 微小伝熱面における沸騰熱伝達に関する研究(OS9 熱・流動)

Pool boiling heat transfer experiments for micro heat transfer surfaces were performed by using ethanol as test fluid. The experiments were conducted at 0.101 MPa. The test heat transfer surfaces were 20, 10, 7, 5, 3, 2 and 1 mm in diameter. The test heat transfer surfaces were made of copper. Electric heaters were attached at the back end of the test heat transfer surface. Boiling behavior was also recorded with a high speed video camera. The overall trend of the boiling heat transfer was well expressed with the Rohsenow correlation. As the diameter of the boiling surface became smaller, the boiling heat transfer was enhanced and the critical heat flux increased.