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Interconnected microchannels (IMCs) in flow boiling have the advantages of optimized heat transfer performance, energy savings and high efficiency, compact size, and strong customizability. They provide new solutions for thermal management and heat transfer at the microscale and have broad application prospects. To further investigate the effect of microchannels with different numbers of transverse sections on the flow boiling heat transfer, we performed numerical simulations on a rectangular microchannel (RMC) and IMCs with 3, 5, and 7 transverse microchannels at high and low mass flux. It was found that fluid experiences similar bubble and slug flow in different numbers of IMCs and the RMC at low mass flux. At a heat flux of q = 90 W/cm2, the downstream regions of the IMCs produce vapor films that span the channels, obstructing the cross-section and weakening the flow exchange between the channels, which lead the heat transfer performance factor of IMC-3, reaching 148.43%, 110.04%, and 116.92% of the RMC, IMC-5, and IMC-7. Under high-quality flux, as the heat flux increases, the heat transfer coefficient increases and the pressure drop decreases due to the existence of lateral microchannels introduced in the interconnected microchannels. Whether at high or low mass flux, structural reasons pertaining to the RMC can easily lead to the accumulation of bubbles and the occurrence of slugs, and the flow boiling instability increases with the increase of heat flux, which leads to a pressure drop and heat transfer performance generally lower than that of IMCs under the same conditions. At q = 120 W/cm2, IMC-7 showed the best heat transfer enhancement. Its heat transfer performance factor was 129.37%, 120.594% and 107.98% of the RMC, IMC-3, and IMC-5, respectively. This article provides theoretical support for the design of interconnected microchannels in thermal management. [ABSTRACT FROM AUTHOR]

The article aims to explore boiling heat transfer in mini-channels with a rectangular cross-section using various fluids (HFE-649, HFE-7000, HFE-7100, and HFE-7200). Temperature measurements were conducted using infrared thermography for the heated wall and K-type thermocouples for the working fluid. The 2D mathematical model for heat transfer in the test section was proposed. Local heat transfer coefficients between the heated wall and the working fluid were determined from the Robin condition. The problem was solved by means of the finite element method (FEM) with Trefftz functions. The values of the heat transfer coefficient that were obtained were compared with the results calculated from Newton's law of cooling. The average relative differences between the obtained results did not exceed 4%. The study included uncertainty analyses for temperature measurements with K- and T-type thermocouples. Expanded uncertainties were calculated using the uncertainty propagation and Monte Carlo methods. Precisely determining the uncertainties in contact temperature measurements is crucial to ensure accurate temperature data for subsequent heat transfer calculations. The results of the heat transfer investigations were compared in terms of fluid temperature, heat transfer coefficients, and boiling curves. HFE-7200 consistently exhibited the highest fluid temperature and temperature differences at boiling incipience, while HFE-7000 demonstrated the highest heat transfer coefficients. HFE-649 showed the lowest heat transfer coefficients. The boiling curves exhibited a typical shape, with a notable occurrence of 'nucleation hysteresis phenomena'. Upon the analysis of two-phase flow patterns, bubbly and bubbly-slug structures were observed. [ABSTRACT FROM AUTHOR]

It is widely established that flow boiling, being a direct cooling technique also employing the latent heat of the fluid, has the potential to be more efficient than being useful in single-phase conventional cooling methods. This results in considerable potential for thermal management in many fields like microelectronics, space technology, thermal power plants, etc. The increasing demand for heat dissipation, consequent to component miniaturization, has pushed the development of new strategies for enhancing heat transfer efficiency, such as employment of functionalized surfaces. This review aims to describe in detail the current status of technology related to flow boiling heat transfer enhancement via micro/nanoscale surface functionalization. Key objectives are an increased nucleation site density and enhanced bubble dynamics. The vast majority of findings show favorable heat transfer performance, evidenced by an earlier onset of boiling (ONB), an improved flow boiling heat transfer coefficient (HTC), and an ameliorated critical heat flux (CHF). Increased pressure drop is a serious concern in certain application cases. Nanoscale textures mainly enhance capillary wicking to nucleation sites, thus being more effective in combination with microscale textures that define fixed nucleation sites. Degradation effects need to be more thoroughly and systematically characterized for application cases. Extra effects related to the manufacturing process can be easily overlooked, but one should be aware of their possible existence when drawing conclusions. Finally, the implementation of enhanced surfaces in mainstream applications is hindered by the absence of general predictive design tools for different channel configurations/materials, fluids, and operating conditions. A more universal understanding of the basic mechanisms involving texture geometry is needed in this aspect. [ABSTRACT FROM AUTHOR]

The use of a micro heat sink is an effective means of solving the problem of high-power chip heat dissipation. Diamond/Cu composites exhibit high thermal conductivity and a linear thermal expansion coefficient that is compatible with semiconductor materials, rendering them ideal micro heat sink materials. The aim of this study was to fabricate diamond/Cu and Cu separately as heat sinks and subject them to flow boiling heat transfer experiments. The results indicate that the diamond/Cu heat sink displayed a decrease in wall superheat of 10.2–14.5 °C and an improvement in heat transfer coefficient of 38–51% compared with the Cu heat sink under identical heat fluxes. The heat sink also exhibits enhanced thermal uniformity. Secondary diamond particles are incorporated into the gaps of the main diamonds, thereby constructing a three-dimensional heat conduction network within the composite material. The diamond network enhances the internal heat flux of the material while also creating more nucleation sites on the surface. These increase the boiling intensity of the diamond/Cu heat sink, leading to better heat transfer performance. By combining the transient thermal model with computational fluid dynamics, a heat transfer model based on the diamond/Cu heat sink is proposed. The efficient heat dissipation capability of diamond/Cu heat sinks can lower the working temperature of microelectronic devices, thereby improving device performance and reliability during operation. [ABSTRACT FROM AUTHOR]

The evaporation of liquid hydrocarbon n-heptane is discussed in detail with experimentation and numerical techniques. A maximum wall temperature of 1050 K was reported during an experimental process with a two-phase flow that was stable and had a prominent meniscus at a small fuel flow rate (FFR) ≤ 10 µL/min. At medium to high FFR (30–70 µL/min), the flow field was unstable, with nucleating bubbles and liquid droplets inside the microtube and the maximum temperature recorded was 850 K for 70 µL/min. For the numerical model, the temperature of the wall was used as a boundary condition. Using the numerical model, the evaporative flux at the meniscus, pressure drop, pressure oscillation, and heat transfer coefficient (HTC) were investigated. A single peak in HTC was obtained at a low fuel flow rate, while multiple peaks were obtained for high FFR. At low FFR, the pressure peak was observed to be 102.4 KPa, whereas at high FFR, the pressure peak increased to 105.5 KPa. This shows a 2% increase in pressure peak with an increase in FFR. Similarly, when the FFR increased from 5 µL/min to 70 µL/min, the pressure drop increased from 500 Pa to 2800 Pa. The high amplitude of pressure drops and a high peak of HTC were found, which depend on the mass flow rate. The coefficient of variation for pressure drop depends mainly on the fuel flow rate. [ABSTRACT FROM AUTHOR]

This paper studies the evaporative heat transfer characteristics of R744 at low temperatures in a horizontal smooth tube as a cascade refrigeration system (CRS) among hybrid cascade refrigeration systems (HCRSs). There is a lack of research on the low-temperature evaporative heat transfer characteristics of R744 under the operating conditions of evaporators used in actual CRSs used in supermarkets. Therefore, this study aims to provide basic data on the evaporative heat transfer characteristics of R744 in the evaporators of refrigerators used in supermarkets. The tube used in the evaporation experiment conducted herein was a smooth horizontal copper tube with an inner diameter and length of 11.46 mm and 8000 mm, respectively. The experimental parameters were as follows: heat fluxes of 11.8–21.3 kW/m2, mass fluxes of 76.3–175.1 kg/(m2·s), and saturation temperatures of −49.8–−30.2 °C. The main results are summarized as follows: the evaporative heat transfer coefficient of R744 can be predicted well by using the correlation formula of Chen at the evaporation temperature of −40 °C in the CRS. [ABSTRACT FROM AUTHOR]

The trend of miniaturisation in recent decades has led to the development of compact electronic devices. The reduction in the required dimension leads to the exponential rise in the heat flux dissipated from such a system. A proper thermal management system is necessary to keep the temperature of a computer chip's junction within acceptable limits and maintain its performance. Flow boiling modification using straight fins in microchannels has proven to be an effective passive enhancement of the cooling system. The core interest of this research is figuring out the optimal configuration of the fin shapes and configurations. Hence, it is crucial to gain a comprehensive understanding of the flow boiling phenomenon to establish a more general approach. In this study, the boiling heat transfer performance of fin microchannels with various shapes and dimensions is investigated experimentally. The study has shown that the choice of fin geometry has a significant impact on the thermal performance of a heat transfer system. Specifically, the results indicate that a rectangular cross-section fin performs better than a trapezoidal one with the same fin gap. The rectangular cross-section fin exhibits the highest heat transfer coefficient of 5066.84 W/m2∙K, outperforming the trapezoidal fin in terms of heat transfer capability. As the hydraulic diameter reduces, the thermal boundary layer becomes denser, providing a more distributed saturated region. This leads to the increase in the heat transfer coefficient up to 22.5% and 17.1% for rectangular and trapezoidal fins, respectively. Additionally, the efficiency analysis shows that, albeit increasing the mass flux and reducing the gap increase the average cooling performance, but the pressure drop jumps up to 48%, reducing the efficiency of the heat removal system. [ABSTRACT FROM AUTHOR]

This review study examines flow boiling heat transfer in micro-fin tubes using mixed and pure refrigerants with zero ozone-depleting potential (ODP) and minimal global warming potential (GWP). This investigation focuses on the extraordinary relationship between heat transfer coefficients (HTCs) and vapor quality. Since the introduction of micro-fin heat exchanger tubes over 30 years ago, refrigerant-based cooling has improved significantly. Air conditioning and refrigeration companies are replacing widely used refrigerants, with substantial global warming impacts. When space, weight, or efficiency are limited, micro-fin heat exchangers with improved dependability are preferred. This review article discusses flow boiling concepts. The researchers used several refrigerants under different testing conditions and with varying micro-fin tube parameters. Micro-fin tubes are promising for improved heat transfer techniques. This tube increases the heat transfer area, fluid disturbance, flow speed, and direction owing to centrifugal force and HTC. As the focus shifts to improving heat transfer, pressure drop, mean vapor quality, and practical devices, this subject will grow more intriguing. A radical shift will reduce equipment size for certain traditional heat transfer systems and bring new products using micro-scale technologies. This suggested review effort helps comprehend saturation flow boiling through micro-fin tubes and find the right correlation for a given application. This domain's challenges and future relevance are also discussed. [ABSTRACT FROM AUTHOR]

In recent years, the open microchannel has drawn increasing interest, but severe local dryout limited the heat transfer capability of flow boiling. It was anticipated that nanostructures with exceptional capillary wicking abilities would overcome this problem. In this study, blade-like CuO nanostructures were created in the copper open microchannels to experimentally investigate water flow boiling. Experiments were carried out in nanostructured open microchannels (NMCs), and smooth-surface open microchannels (SMCs), as a comparison, were examined under identical operating conditions. Four main flow patterns, including bubbly flow, slug flow, and two kinds of stratified flow, dominated successively in NMCs and SMCs. Although the flow patterns were similar in NMCs and SMCs, the heat transfer coefficient (HTC) of flow boiling was greatly enhanced by nanostructures under conditions of medium and high heat flux, while the nanostructures' influence on HTC was unnoticeable at low heat flux. At medium and high heat fluxes, the dependence of HTC on heat flux and flow rate indicated the joint contribution of nucleate boiling mechanism and convective evaporation mechanism to heat transfer. The enhanced effect of nanostructures on nucleate boiling and convective evaporation became more prominent as heat flux increased, leading to a higher HTC in NMCs than in SMCs at higher heat flux conditions. [ABSTRACT FROM AUTHOR]

The main aim of this work was to provide heat transfer calculations of flow boiling in mini-channels with an application for the Trefftz functions. The test section comprised five parallel mini-channels with a depth of 1 mm, with a common heated wall. For the estimation of the temperature uncertainty, during the experiment the temperature measurement was performed with the use of K-type thermoelements and an infrared camera in two mini-channels simultaneously. According to the Guide to the Expression of Uncertainty in Measurement, the Monte Carlo method is a practical alternative to the GUM uncertainty framework. Since the uncertainty components are not approximately the same magnitude, the Monte Carlo method was indicated to estimate the uncertainty of the surface temperature measurement. The results obtained from this simulation method were compared with the results of the computation related to the uncertainty propagation method. The results of both methods of temperature measurement were found to be consistent. The results of the statistical analysis were used to describe heat transfer calculations. The heat transfer investigations concerning the subcooled boiling region were performed during the other experiment. The local heat transfer coefficients on the contact surface between the working fluid and the heated wall were calculated from the Robin boundary condition. The mathematical model described by the heat equation in the mini-channel wall and by the Fourier-Kirchhoff equation in a flowing fluid leads to an inverse heat transfer problem. This problem was solved using the FEM with the Trefftz-type basis functions. The estimation of temperature uncertainty measurements due to the Monte Carlo method was included in the final results of the heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Traditional microchannel needs to face the flow-reversal difficulty in high heat fluxes due to limited space. It results in large pressure and temperature fluctuation. Porous microchannels arouse more interest to provide a new solution to this problem. Flow boiling experiments in porous microchannels with PFA were investigated. Porous microchannels were sintered by 10 μm (or 30 μm) spherical copper particles with pore-forming agent (Na2CO3, 60–90 μm). Porous microchannels were composed of 23 parallel porous microchannels with 600 μm in width and 1200 μm in depth.The addition of PFA (pore-forming agent) could increase the sample porosity. For Q10 series, sample porosities increase from 20.4% to 52.9% with the PFA percentage change from 0% to 40%, while for the Q30 series they increase from 26.6% to 47.5%. Experimental results showed the boiling heat transfer coefficient (HTC) reached the maximum at the moderate porosity for both Q10 and Q30 series. Too large or too small porosity would degrade boiling heat transfer performance. It demonstrated that there existed an optimal range of PFA content for sintered microchannels. PFA content has a minor effect on the average pressure drop and would not cause the rapid increase in flow resistance. Visual observation disclosed that the sample porosity would affect the pressure instability significantly. The sample with moderate porosity showed periodic pressure fluctuation and could establish rhythmical boiling. Particle size also exerted a certain influence on the boiling heat transfer performance. Q30 series could achieve higher HTC and CHF (Critical heat flux) than Q10 series. This is attributed to the larger ratio of layer-thickness-to-particle-size (δ/d) for Q10-series samples. [ABSTRACT FROM AUTHOR]

Even though two-phase heat transfer of refrigerants in minichannel heat sinks has been studied extensively, there is still a demand for improvements in overall thermal performance of miniature heat transfer exchangers. Experimental investigation and sophisticated heat transfer calculations with respect to heat transfer devices are still needed. In this work, a time-dependent experimental study of subcooled boiling was carried out for FC-72 flow in a heat sink, comprising of five asymmetrically heated minichannels. The heater surface temperature was continuously monitored by an infrared camera. The boiling heat transfer characteristics were investigated and the effect of the mass flow rate on the heat transfer coefficient was studied. In order to solve the heat transfer problem related to time-dependent flow boiling, two numerical methods, based on the FEM were applied, and based on the Trefftz functions (FEMT) and using the ADINA program. The results achieved with these two calculation methods were explored with an emphasis on the impact of the mass flow rate (range from 5 to 55 kg/h) on the resulting heat transfer coefficient. It was found that, with increasing mass flow, the heat transfer coefficient increased. Good agreement was found between the heat transfer coefficients, determined according to two numerical methods and the simple 1D calculation method. [ABSTRACT FROM AUTHOR]

This work aimed to study the intensification of flow boiling heat transfer and critical heat flux (CHF) under conditions of highly reduced pressures due to a modification of the inner wall surface of a mini-channel. Such research is relevant to the growing need of high-tech industries in the development of compact and energy-efficient heat exchange devices. We present experimental results of the surface modification effect on hydrodynamics and flow boiling heat transfer, including data on the CHF. A description of the experimental stand and method for modifying the test mini-channel is also presented. The studies were carried out with freon R-125 in a vertical mini-channel with a diameter of 1.1 mm and a length of 50 mm, in the range of mass flow rates from G = 200 to 1400 kg/(m2s) and reduced pressures between pr = p/pcr = 0.43 and 0.56. The maximum surface modification effect was achieved at a reduced pressure of pr = 0.43, the heat transfer coefficient increased up to 110%, and the CHF increased up to 22%. [ABSTRACT FROM AUTHOR]

A comparative analysis of the flow boiling characteristics of R450A, R515B, and R1234ze(E) inside a 7.0 mm OD microfin tube was performed. The mass velocity was explored from low values, starting from 50 kg m−2 s−1, up to 400 kg m−2 s−1, and the heat flux was tested in the range of 10–50 kW m−2, keeping a constant saturation temperature of 30 °C at the inlet of the test section. R515B and R1234ze(E) showed similar values of the heat transfer coefficient and frictional pressure gradient under all the investigated working conditions. R450A showed lower values of the heat transfer coefficient, especially at low heat flux and high mass velocity, compared to R515B and R1234ze(E), but the gap of the heat transfer performance between the three fluids reduced at high heat flux. The frictional pressure drops of R450A were generally lower than those of R515B and R1234ze(E). In the end, some correlations for the evaluation of both the heat transfer coefficient and the pressure drop were selected, and the estimated values were compared against the experimental counterpart. Furthermore, the effects of nucleate boiling and of convective boiling, as well as of the temperature glide for R450A, were analyzed and estimated. [ABSTRACT FROM AUTHOR]

This study presents an extensive evaluation of heat transfer characteristics, flow patterns, and pressure drop for saturation pressures ranging from 460–660 kPa in a horizontal smooth tube of 5 mm internal diameter using R134a as the working fluid. The effect of saturation pressures for mass fluxes of 150–300 kg/m2s and heat fluxes of 8.26–23.3 kW/m2 which are typical of refrigeration and air conditioning applications are also investigated. Flow patterns observed during the study are predicted with a well-known flow pattern map of Wojtan et al. The experimental results are compared with seven (7) correlations developed based on different theories to find which correlation best predicts the experimental data. The results show that, at low mass flux, increasing saturation pressure results in an increased heat transfer coefficient. This effect is more pronounced in the low vapor quality region and the dominant mechanism is nucleate boiling. At high mass flux, increasing saturation pressure leads to an insignificant increase in the heat transfer coefficient. At this high mass flux but low heat flux, the heat transfer coefficient increases with vapor quality, indicating convective boiling dominance. However, for high heat flux, the heat transfer coefficient is linear over vapor quality, indicating nucleate boiling dominance. Pressure drop is observed to decrease with increasing saturation pressure. Increasing saturation pressure increases the vapor quality at which the flow pattern transitions from intermittent flow to annular flow. The flow patterns predicted are a mixture of slug and stratified wavy and purely stratified wavy for low mass fluxes. For increased mass fluxes, the flow patterns predicted are slug, intermittent, annular, and dryout. Cooper's model was the best predictor of the experimental data and the trend of heat transfer coefficient. [ABSTRACT FROM AUTHOR]

The process with change of phase during flow in mini-channels plays a significant role in many industrial applications, such as microelectronics. Furthermore, methods for heat transfer intensification during flow in channels of small cross-section are still being sought. In this work, studies of the effect of using a modified heated surface on intensification boiling heat transfer in rectangular mini-channels during upward and downward flow are performed. The test section of a group of seven parallel mini-channels 1 mm deep was investigated during the subcooled and saturated flow boiling of FC-72. The temperature of the outer heated wall surface was measured using an infrared camera. During the experiments, two-phase flow structures were captured with a quick camera. Local heat transfer coefficients at the contact surface between the working fluid and the heated surface were determined with the use of a one-dimensional calculation method. To present the results, local temperature measurements and heat transfer coefficients, boiling curves and two-phase flow patterns are shown and analyzed. The results concerning two directions of vertical flow along mini-channels are discussed. Several modified heated surfaces and one smooth were tested for comparison. The main objective was to find out how the modified surface of the heated wall can intensify boiling heat transfer with upward and downward refrigerant flow in mini-channels of rectangular cross-section. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model is established to study the flow boiling heat transfer characteristics of the liquid film in a rotating pipe, and the effectiveness of the model is verified by a comparison between the numerical results and the experimental results. The effects of rotational speed, heat flux, and Coriolis force on the characteristics of heat transfer of the rotating liquid film are investigated. The conclusions are drawn as follows: (1) The convection of the rotating liquid film is enhanced while the nucleate boiling in the rotating liquid film is inhibited by the increase in the rotational speed; (2) With the influence of these two factors, the heat transfer coefficient increases with centrifugal acceleration increasing from 20 g to 40 g, then decreases with centrifugal acceleration increasing from 40 g to 120 g; (3) The turbulent intensity of the flow with Coriolis force is obviously increased compared to that without Coriolis force when the centrifugal acceleration ranges from 20 g to 80 g, which shows no increase at higher centrifugal accelerations when the turbulence is sufficiently strong. The Coriolis force also has an impact on the heat transfer coefficient of the liquid film, which should not be ignored when studying the boiling heat transfer of a rotating liquid film. [ABSTRACT FROM AUTHOR]

In this study, the evaporation heat transfer characteristics of an intermediate temperature of R744 in a smooth horizontal tube, when operating as an indirect refrigeration system (IRS) among hybrid cascade refrigeration systems (HCRSs), are evaluated. Studies on the characteristics of intermediate-temperature evaporation heat transfer under the operating conditions of evaporators used in actual refrigeration systems, such as IRS, cascade refrigeration systems, and HCRS, used in supermarkets are lacking. Thus, this study provides basic data on the characteristics of evaporation heat transfer of R744 in the evaporators of refrigerators used in supermarkets. The tube employed to the evaporation experiment in this study was a horizontal smooth copper tube with a length of 8000 mm and an inner diameter of 11.46 mm. The experimental variables were measured over a wide range of mass flux of 200–500 kg/(m2·s), heat flux of 10–40 kW/m2, and saturation temperature of −40–0 °C. The main results are summarized as follows: (1) The application of Kandlikar's correlation formula at an evaporation temperature of −20 °C in an IRS helps in a good prediction of the R744 evaporation heat transfer coefficient. (2) The pressure drop according to the heat and mass flux showed the same heat transfer coefficient trend, but the pressure drop at saturation temperature was different from the trend of heat transfer coefficient. [ABSTRACT FROM AUTHOR]

This paper presents the results of research on heat transfer during fluid flow in a heat exchanger with two rectangular mini-channels. There was Fluorinert FC-72 flow, heated by the plate in the hot mini-channel, and co-current flow of distilled water in the cold mini-channel. Both fluids were separated by the copper plate. A thermal imaging camera was used to measure the temperature distribution of the outer surface of the heated plate. The purpose of the calculations was to determine the heat transfer coefficients at the contact surfaces: the heated plate—FC-72 and FC-72—the copper plate. Two mathematical models have been proposed to describe the heat flow. In the 1D approach, only the heat flow direction perpendicular to the fluid flow direction was assumed. In the 2D model, it was assumed that the temperature of the heated plate and FC-72 and the copper plate meet the appropriate energy equation, supplemented by the boundary conditions system. In this case, the Trefftz functions were used in numerical calculations. In the 1D model, the heat transfer coefficient at the interface between FC-72 and the copper plate was determined by theoretical correlations. The analysis of the results showed that the values and distributions of the heat transfer coefficient determined using both models were similar. [ABSTRACT FROM AUTHOR]

The heat transfer and pressure drop characteristics of R290 flow boiling in a corrugated tube were investigated through computational fluid dynamics (CFD) in this study. We established a model of flow boiling in a corrugated tube with different corrugated structures (rectangular and circular corrugations) and validated the model using the Liu–Winterton and Xu–Fang empirical equations. The heat transfer coefficient (HTC) and pressure drop were obtained at a mass flow rate of 0.04–0.2 kg/s and a water inlet temperature of 310–330 K. The results show that the HTC and the drop in the pressure of the corrugated tubes both obviously increased compared with a smooth tube as the mass flow rate increased. The HTC decreased for the three tubes as the water inlet temperature increased, while the drop in pressure slightly increased for the three tubes. Moreover, the corrugated structure was found to significantly enhance the heat transfer; the heat transfer enhancement factor ( E 1 ) of the corrugated tube with the rectangular corrugations and the corrugated tube with the circular corrugations was 2.01–2.36 and 1.67–1.98, respectively. The efficiency index (I) for both the rectangular corrugated pipe and the circular corrugated pipe was greater than 1 (1.05–1.24 and 1.13–1.29, respectively). The application of corrugated tubes with round and rectangular corrugations can reduce the heat transfer area required for the exchange of heat and, thus, reduce the cost. [ABSTRACT FROM AUTHOR]

In this paper, the impact of different factors on the flow boiling of R290 and R22 in double-concentric pipes are investigated through CFD numerical simulations. The numerical studies are performed by changing the inner tube diameter in the range of 3 to 7 mm, the refrigerant velocity between 1 and 5 m/s, the water velocity between 1 and 10 m/s and the saturation temperature in the range of 276 to 283 K. The heat transfer coefficient (HTC), pressure drop and exergy destruction of R290 are determined. The results show that HTC, pressure drop and exergy destruction are significantly impacted by the pipe diameter and the refrigerant velocity, but slightly impacted by water velocity and saturation temperature. Moreover, the exergy loss and pressure drop of R290 are 11.8–13.3% and 4.3–10.2% lower than those of R22. R290 has a lower energy loss than R22 in the evaporation process in the double-concentric pipe. However, the HTC of R290 is 57.3–59.7% lower than that of R22. The HTC of R290 can be optimized by increasing the pipe diameter or the R290 velocity. [ABSTRACT FROM AUTHOR]

To protect the environment, a new low-GWP refrigerant R1234ze(E) was created to substitute R134a. However, its flow boiling performances have not received sufficient attention so far, which hinders its popularization to some extent. In view of this, an experimental investigation was carried out in a 1.88 mm horizontal circular minichannel. The saturation pressures were maintained at 0.6 and 0.7 MPa, accompanied by mass flux within 540–870 kg/m2 s and heat flux within 25–65 kW/m2. For nucleate boiling, a larger heat flux brings about a larger heat transfer coefficient (HTC), while for convective boiling, the mass flux and vapor quality appear to take the lead role. The threshold vapor quality of different heat transfer mechanisms is around 0.4. Additionally, larger saturation pressure results in large HTC. As for the frictional pressure drop (FPD), it is positively influenced by mass flux and vapor quality, while negatively affected by saturation pressure, and the influence of heat flux is negligible. Furthermore, with the measured data, several existing correlations are compared. The results indicate that the correlations of Saitoh et al. (2007) and Müller-Steinhagen and Heck (1986) perform best on flow boiling HTC and FPD with mean absolute deviations of 5.4% and 10.9%. [ABSTRACT FROM AUTHOR]

Heat transfer under flow boiling is better in a rectangular channel filled with open-cell metal foam than in an empty channel, but the high pressure drop is a drawback of the empty channel method. In this study, various types of metal foam insert configurations were tested to reduce the pressure drop while maintaining high heat transfer. Specifically, we measured the boiling heat transfer and pressure drop of a two-phase vertical upward flow of R245fa inside a channel. To measure the pressure and temperature differences of the metal foam, differential pressure transducers and T-type thermocouples were used at both ends of the test section. While the saturation pressure was kept constant at 5.9 bar, the steam quality at the inlet of the test section was changed from 0.05 to 0.99. The channel height, moreover, was 3 mm, and the mass flux ranged from 133 to 300 kg/m2s. The two-phase flow characteristics were observed through a high-speed visualization experiment. Heat transfer tended to increase with the mean vapor quality, and, as expected, the fully filled metal foam channel offered the highest thermal performance. The streamwise insert pattern model had the lowest heat transfer at a low mass flux. However, at a higher mass flux, the three different insert models presented almost the same heat transfer coefficients. We found that the streamwise pattern model had a very low pressure drop compared to that of the spanwise pattern models. The goodness factors of the flow area and the core volume of the streamwise patterned model were higher than those of the full-filled metal foam channel. [ABSTRACT FROM AUTHOR]

This paper reports the results of heat transfer during refrigerant flow in rectangular mini-channels at stationary conditions. The impacts of selected parameters on boiling are discussed, i.e., thermal and flow parameters, dimensions and orientation of the channels. Four refrigerants (FC-72, HFE-649, HFE-7000 and HFE-7100) were used as the working fluid. Research was carried out on the experimental set-up with the test section with a single rectangular mini-channel of 180 mm long and with a group of five parallel mini-channels, each 32 mm long. The temperature of the mini-channel's heated wall was measured by infrared thermography. Local values of the heat transfer coefficient at the contact surface between the fluid and the plate were calculated using the 1D mathematical method. The results are presented as the relationship between the heat transfer coefficient and the distance along the mini-channel length and boiling curves. Two-phase flow patterns are shown. Moreover, the results concerning various refrigerants and the use of modified heater surfaces are discussed. The main factors influencing the heat transfer process were: mini-channel inclination to the horizontal pane (the highest heat transfer coefficient at 270° and 0°), using modified heater surfaces (especially electroerosion texturing and vibration-assisted laser No. 2 texturing) and working fluids (FC-72 and HFE-7000). [ABSTRACT FROM AUTHOR]

In this paper experiments have been conducted to investigate the flow boiling and heat transfer characteristics in microchannels with three different surface wettability. Three types of microchannels with a super-hydrophilic surface (θ ≈ 0°), a hydrophilic surface (θ = 43°) and an untreated surface (θ = 70°) were prepared. The results show that the average heat transfer coefficient of a super-hydrophilic surface microchannel is significantly higher than that of an untreated surface microchannel, especially when the mass flux is high. The visualization of the flow patterns states that the number of bubble nucleation generated in the super-hydrophilic microchannel at the beginning of the flow boiling is significantly more than that in the untreated microchannel. Through detailed analysis of the experimental data, flow patterns and microchannel surface SEM images, it can be inferred that the super-hydrophilic surface microchannel has more active nucleation cavities, a high nucleation rate and a large nucleation number, a small bubble departure diameter and a fast departure frequency, thereby promoting the flow and heat transfer in the microchannel. In addition, through the force analysis of the vapor-liquid interface, the mechanism that the super-hydrophilic microchannel without dryout under high heat flux conditions is clarified. [ABSTRACT FROM AUTHOR]

The intensification of heat transfer using two-phase boiling flow in mini-channels is widely used to dissipate the high heat fluxes in miniaturized electronic devices. However, the process itself is not fully recognized and still requires experimental studies and developing computation methods appropriate for them. The main aim of this work was the mathematical modeling of time-dependent heat transfer process in FC-72 flow boiling in a mini-channel heat sink with five parallel mini-channels of 1 mm depth. Channels have an asymmetrically heated wall while its outer temperature was measured by infrared thermography. The opposite wall of the mini-channels was transparent, helping to record flow patterns due to a high-speed digital camera. The objective of the numerical calculations was to determine the heat transfer coefficient on the wall-fluid contact surface from the Robin boundary condition. The problem was solved using methods based on the Trefftz-type functions. Three mathematical methods were applied in calculations: the FEM with Trefftz type basis functions, the Classical Trefftz Method, and the Hybrid Picard-Trefftz Method. The results were compared with the values of the heat transfer coefficient obtained from theoretical correlations from the literature. [ABSTRACT FROM AUTHOR]

This work focuses on subcooled boiling heat transfer during flow in a minichannel heat sink with three or five minichannels of 1 mm depth. The heated element for FC-72 flowing along the minichannels was a thin foil of which temperature on the outer surface was measured due to the infrared thermography. The test section was oriented vertically or horizontally. A steady state heat transfer process and a laminar, incompressible flow of the fluid in a central minichannel were assumed. The heat transfer problem was described by the energy equations with an appropriate system of boundary conditions. Several mathematical methods were applied to solve the heat transfer problem with the Robin condition to determine the local heat transfer coefficients at the fluid/heated foil interface. Besides the 1D approach as a simple analytical method, a more sophisticated 2D approach was proposed with solutions by the Trefftz functions and ADINA software. Finite element method (FEM) calculations were conducted to find the temperature field in the flowing fluid and in the heated wall. The results were illustrated by graphs of local heated foil temperature and transfer coefficients as a function of the distance from the minichannel inlet. Temperature distributions in the heater and the fluid obtained from the FEM computations carried out by ADINA software were also shown. Similar values of the heat transfer coefficient were obtained in both the FEM calculations and the 1D approach. Example boiling curves indicating nucleation hysteresis are shown and discussed. [ABSTRACT FROM AUTHOR]

Flow boiling heat transfer in microchannels can provide a high cooling rate, while maintaining a uniform wall temperature, which has been extensively studied as an attractive solution for the thermal management of high-power electronics. The depth-to-width ratio of the microchannel is an important parameter, which not only determines the heat transfer area but also has dominant effect on the heat transfer mechanisms. In the present study, numerical simulations based on the volume of fraction models are performed on the flow boiling in very deep microchannels. The effects of the depth-to-width ratio on the heat transfer coefficient and pressure drop are discussed. The bubble behavior and heat transfer characteristics are analyzed to explain the mechanism of heat transfer enhancement. The results show the very deep microchannels can effectively enhance the heat transfer, lower the temperature rise and show promising applications in the thermal management of high-power electronics. [ABSTRACT FROM AUTHOR]

In this paper, the issue of flow boiling heat transfer in an annular minigap was discussed. The main aim of the paper was determining the boiling heat transfer coefficient at the HFE-649 fluid–heater contact during flow along an annular minigap. The essential element of the experimental stand was a test section vertically oriented with the minigap 2 mm wide. Thermocouples were used to measure the temperature of the heater and fluid at the inlet and the outlet to the minigap. The mathematical model assumed that the fluid flow was laminar and the steady–state heat transfer process was axisymmetric. The temperatures of the heated surface and of the flowing fluid were assumed to fulfill energy equations with adequate boundary conditions. The problem was solved by the Trefftz method. The local heat transfer coefficients at the fluid–test surface interface were calculated due to the third kind boundary condition at the saturated boiling. Graphs were used to illustrate: the measurement of the heater surface temperature, 2D temperature distributions in the pipe and fluid, and the heat transfer coefficient as a function of the distance from the minigap inlet. The measurement uncertainties and accuracy of the heat transfer coefficient determination were estimated. [ABSTRACT FROM AUTHOR]

An experimental investigation was conducted to explore the flow boiling heat transfer characteristics of refrigerants R134A and R410A inside a smooth tube, as well as inside two newly developed surface-enhanced tubes. The internal surface structures of the two enhanced tubes are comprised of protrusions/dimples and petal-shaped bumps/cavities. The equivalent inner diameter of all tested tubes is 11.5 mm, and the tube length is 2 m. The experimental test conditions included saturation temperatures of 6 °C and 10 °C; mass velocities ranging from 70 to 200 kg/(m2s); and heat fluxes ranging from 10 to 35 kW/m2, with inlet and outlet vapor quality of 0.2 and 0.8. It was observed that the enhanced tubes exhibit excellent flow boiling heat transfer performance. This can be attributed to the complex surface patterns of dimples and petal arrays that increase the active heat transfer area; in addition, more nucleation sites are produced, and there is also an increased interfacial turbulence. Results showed that the boiling heat transfer coefficient of the enhanced surface tubes was 1.15–1.66 times that of the smooth tubing. Also, effects of the flow pattern and saturated temperature are discussed. Finally, a comparison of several existing flow boiling heat transfer models using the data from the current study is presented. [ABSTRACT FROM AUTHOR]