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In some thermally driven two-phase natural circulation systems, bubble pumps serve as the key driving powers for the cycles. Recently a type of distribute heating bubble pump generator (BPG) is gradually receiving attention due to its compact structure and great potentials to utilize solar energy and low-grade waste heat recovery. The BPG provides a variety of promising features (e.g., passive heat transfer, enhanced reliability), which can benefit the advancing of heat transfer technology. For the primary study, we performed an experiment in a distributed heating BPG. Through utilizing multiple lift tubes and partial visualization configurations, it provides accesses to observe the flow pattern transition and monitor the flow instability, and thus to explore some of the underlying mechanisms affecting BPG performance. Results showed that heat input and immersion height were crucial parameters to enable the operation of distribute heating BPG. With low heat input or high inlet water subcooling level, the flow within the pump was unstable with intermittent flow interruptions. As the heat input increased, the fluid flow became more stable, the vapor generation increased linearly, while the lifted liquid flow rate initially increased then decreased. Correspondingly, the flow pattern at the outlet section of lift tubes gradually changed from slug flow to churn flow, and then to annular flow. The higher of the immersion was, the higher heat input was needed for the flow pattern transition. It was in the churn flow regime at the outlet of lift tubes for the BPG to lift a maximum liquid. At lower immersion level, liquid reflux in the lift tubes was obvious and affected the flow stability as well as the lifting performance. At higher immersion level, the fluid flow was more stable and faster, which lifted more liquid while generated less vapor depending on the inlet subcooling. In general, the BPG showed better performance (both the lifted liquid and vapor generation increased) at smaller inlet subcooling level or lower system pressure. This study highlights the flow pattern evolution and flow stability, which is helpful to the reliable design and effective operation of the distributed heating BPG.

In this study, we experimentally investigate of a two-phase natural circulation loop that functions as a passive containment cooling system (PCCS). The experimental apparatus comprises two loops: a hot loop, for simulating containment under severe accidents, and a natural circulation loop, for simulating the PCCS. The experiment is conducted by controlling the pressure and inlet temperature of the hot loop in the range of 0.59–0.69 MPa (abs) and 119.6–158.8 °C, respectively. The heat balance of the hot loop is established and compared with a natural circulation loop to assess the thermal reliability of the experimental apparatus, and an additional system is installed to measure the vapor mass flow rate. Furthermore, the thermal–hydraulic characteristics are considered in terms of a temperature, mass flow rate, heat transfer coefficient (HTC), etc. The flow rate of the natural circulation loop is induced primarily by flashing, and a distortion is observed in the local HTC because of the fully develop as well as subcooled boiling. As a result, we present the amount of heat capacity that the PCCS can passively remove according to the experimental conditions and compared the heat transfer performance using Chen's and Dittus-Boelter correlation.

Steam-assisted-gravity-drainage (SAGD) is currently an effective technology for heavy oil recovery. The pre-heating stage is important to affect the overall process of SAGD development. In this paper, flow and heat transfer model in vertical well section and horizontal well section and thermal connectivity judgment model during cyclic pre-heating stage were established. Model calculation results are in good agreement with actual measurement results. The effects of steam injection rate and steam quality on the distribution of steam parameters along the whole wellbore and the distribution of heat transfer were compared. Besides, the optimization of steam injection parameter combination was provided. Ultimately, the effect of horizontal well spacing on pre-heating thermal connectivity was discussed. The results show that: With the increase of steam injection rate and steam injection quality, the pressure and temperature drop of the fluid increase. Wellhead required steam injection pressure and temperature increases and even superheated steam appear with the increase of steam injection rate and steam injection quality. The position where the pre-heating backflow turns from wet steam to subcooled water gradually shifts to the wellhead. With the increase of steam injection rate and steam injection quality, the heat transfer between annulus and reservoir decreases, but the uniformity of pre-heating in the horizontal section increases. The time required for cyclic pre-heating to form thermal communication between double wells increases significantly as the horizontal well spacing increases. To simultaneously integrate the uniformity and economy of pre-heating on the oilfield site, the steam injection quality at the heel end of the horizontal section of 0.7, the steam injection rate of 80 t/d, and the horizontal well spacing of 5 m were recommended.

A visualized experimental test rig of flow boiling in high temperature thermal storage module was presented. The effects of high-temperature sensible heat thermal storage boundary condition on the oscillation characteristics at the onset of dynamic instability were investigated in this work. The flow boiling tests were carried out in a 1.6-meter-long horizontal tube with an inner diameter of 16.0 mm, which was embedded in a sensible thermal storage module using solid graphite. A series of thermal storage temperatures (200–340 °C), inlet subcooling degrees (5–45 °C) and mass flow rates (0–593.5 kg/m2s) were applied. Based on the dimensionless stability map, an early onset of dynamic instability was observed in comparison with the traditional constant heat flux condition. Meanwhile, both the pressure drop oscillations (PDO) and density wave oscillations (DWO) were observed. Based on the development of flow pattern and heat transfer distribution characteristics, the regulation mechanisms of the high-temperature thermal storage boundary on the development of dynamic instability were analyzed. The results show that the characteristics of dynamic instability are highly related to the development of flow pattern. The thermal storage boundary has affected the development of flow pattern by determining the wall superheat. The comprehensive effects of superheat, subcooling degree and mass flux on the development of flow pattern were also interpreted.

In this study, the two-phase pressure drop in a one-sided high-heat-loaded screw tube was analyzed under subcooled flow boiling conditions. When the loaded heat flux is gradually increased under one-sided heating conditions, although the pressure drop changes slightly in the single phase, it starts to increase when loading above the onset of significant void (OSV) heat flux. The effect of system parameters on the two-phase pressure drop in relation to the change of heat transfer mechanism or fluid properties was analyzed according to each variable change. In addition, this study evaluated the prediction performance of existing two-phase pressure drop multiplier correlations developed under subcooled flow boiling conditions. Because most existing correlations are based on a smooth channel with a small diameter, they tend to overpredict the experimental values. The correlation proposed by Ramesh showed the lowest average error rate (31.94%); however, the results were underpredicted for high-heat-flux conditions. To obtain more accurate results, we developed a Python code using an artificial intelligence regression method to proceed with the optimization process based on the experimental values. Using this code, we developed a novel two-phase pressure drop multiplier correlation for a one-side-heated screw tube that can be used in fusion divertors under high-heat-load conditions.

Lubricant-infused surfaces (LISs) can promote stable dropwise condensation and improve heat transfer rates due to a low nucleation free-energy barrier and high droplet mobility. Topographical differences in the oil surface cause water microdroplets to rigorously self-propel long distances, continuously redistributing the oil film and potentially refreshing the surface for re-nucleation. Using high-speed microscopy, we reveal that during water condensation on LISs, the smallest visible droplets (diameter ~ 1um, qualitatively representing nucleation) predominantly emerge in oil-poor regions due to a smaller thermal activation barrier. Considering the significant heat transfer performance of microdroplets (< 10um) and transient characteristic of microdroplet movement, we compare the apparent nucleation rate density and water collection rate for LISs with oils of different viscosity and a solid hydrophobic surface at a wide range of subcooling temperatures. Generally, the lowest lubricant viscosity leads to the highest nucleation rate density. We characterize the length and frequency of microdroplet movement and attribute the nucleation enhancement primarily to higher droplet mobility and surface refreshing frequency. Interestingly and unexpectedly, hydrophobic surfaces outperform high-viscosity LISs at high subcooling temperatures, but are generally inferior to any of the tested LISs at low temperature differences. To explain the observed non-linearity between LISs and the solid hydrophobic surface, we introduce two dominant regimes that influence the condensation efficiency: mobility-limited and coalescence-limited. Our findings advance the understanding of dynamic water-lubricant interactions and provide new design rationales for choosing surfaces for enhanced dropwise condensation and water collection efficiencies., submitted to Langmuir

In this study, the mathematical modeling of microchannel heat exchangers (MCHEs) has been experimentally examined for applications where there is a single-phase refrigerant flow such as pre-heating/cooling, superheating/subcooling. A mathematical simulation model has been developed for louvered fin MCHEs where the working fluid is R600a, which estimates the outlet temperature, total heat transfer capacity and entropy generation of the R600a. To imrpove the accuracy of the model, different mass velocities in the passes and uniform air velocities at the face of MCHE have been taken into consideration in the proposed model. Different from other models in the literature, a calculation system created by two discretization level has been applied to take into account these effects. Non-uniform air velocities have been taken into consideration via dividing the face of the MCHE into airflow regions in the model. Experimental study has been performed to validate the model results. It concluded that the model predicts the outlet temperature with an average absolute deviation within ±10% for all investigated test conditions. It is found that the taking into consideration non-uniform air velocity improves accuracy of the model. The entropy generation mechanisms in the MCHE have been investigated and it has been determined that the contribution of the fluid flow irreversibility to entropy generation is quite low compared to heat transfer irreversibility., Bu çalışmada, özellikle iklimlendirme sistemlerinde ön ısıtma/soğutma, aşırı kızdırma/soğutma gibi tek fazlı soğutkan akışının olduğu uygulamalar için mikrokanal eşanjörlerin matematiksel modellemesi deneysel doğrulamalı olarak ele alınmıştır. Çalışma akışkanının R600a olduğu panjurlu kanatlı mikrokanal eşanjörler için R600a’nın çıkış sıcaklığını, toplam ısı transfer kapasitesini ve entropi üretimini tahmin eden bir matematiksel benzetim modeli geliştirilmiştir. Modelin doğruluk hassasiyetini arttırmak için geçişlerdeki farklı kütle hızları ve uniform olmayan hava hızı modelde dikkate alınmıştır. Bu etkileri dikkate almak için literatürde yer alan diğer modellerden farklı olarak iki seviye ayrıklaştırma ile oluşturulan bir hesaplama sistemi modelde uygulanmıştır. Modelde mikrokanal ısı eşanjörünün ön yüzü hava akış bölgelerine bölünerek uniform olmayan hava hızları dikkate alınmıştır. Model sonuçlarını doğrulamak için deneysel çalışma yapılmıştır. Deneysel doğrulama sonucunda modelin, incelenen tüm test koşulları için çıkış sıcaklığını ±%10 aralığında bir ortalama mutlak sapma ile öngördüğü sonucuna varılmıştır. Mikrokanal ısı eşanjörünün, ısı transfer performansını tahmin etme kabiliyeti, deneylerle değerlendirilmiş, uniform olmayan hava hızının modele dahil etmenin, modelin doğruluk hassasiyetini arttırdığı görülmüştür. Mikrokanal ısı eşanjöründeki entropi üretim mekanizmaları incelenmiş ve akışkan akımı tersinmezliklerinin entropi oluşumuna katkısının, ısı transfer tersinmezliklerine kıyasla oldukça düşük olduğu tespit edilmiştir.

Visualization experiments are carried out to investigate the atomization characteristics of R1336mzz flash spray cooling. The influences of superheat, spray distance, and nozzle orifice diameter on spray cooling performance are analyzed experimentally. As the superheat increases, finer droplets and thinner liquid film are observed; this is helpful to improve the two-phase heat transfer efficiency. Enlarging atomization angle under high superheat is also observed for flash spray cooling, and it benefits for reducing the spray distance. It can be found that when the inlet superheat is 19.8°C and the spray distance is 6 mm, the critical heat flux (CHF) reaches 251 W/cm2 and the maximum heat transfer coefficient (HTC) reaches 37.4 kW/(m2 °C), which are 55% and 11.6% higher than those when the inlet subcooling is 6.9°C and the spray distance is 12 mm, respectively. Using flash spray reduces the spray distance, which benefits for designing compact spray cooling device. In addition, the nozzle orifice diameter has great influence on the cooling performance of flash spray, and the choice of the nozzle depends on the superheat. This study provides a physical insight into the heat transfer enhancement in flash spray cooling.

With the development of industrial technology, heat transfer at the microscale has attracted more and more attention. In this work, 200 µm platinum wire and 150 µm nickel-chromium wire were used as experimental objects, which the power was provided by DC power with the range of 15.6 W to 56.2 W. Distilled water was used as the experimental liquid. Various bubbles on the micro wire were observed and the heat mechanism was analyzed. A variety of bubble attachment phenomena were captured on the 200 µm platinum wire, including the adhesion during bubble detachment, the rotation of small attached bubbles on the surface of large bubble, multiple bubbles circling at the top of the same bubble, and different attached bubble departure phenomena. Marangoni force in the vertical direction triggered the formation of bubble attachment. In addition, the effects of surface tension, adhesion force and buoyancy force on the circling of the bubble were also considered. The analysis of the bubble sweeping on the 150 µm nickel-chromium wire was analyzed. The results showed that the static bubble would interact with the sweeping bubbles on the other side, thereby changing the heat transfer mechanism, which was not discussed in detail before. The bubble jet flow generated by thermocapillary convection on the vertical direction of the bubble surface was the main influencing factor, which would change the microlayer encountered in front of the bubble. The effects of bubble diameter and liquid subcooling on the sweeping velocity were also studied. The results showed that the larger the bubble diameter was, the lower the sweeping velocity would be achieved while the liquid subcooling temperature had less impact on the velocity of sweeping bubble.

The heat transfer process in the condenser is often divided into superheated, two-phase and subcooled zone. But the three-zone model has discontinuities of the heat transfer coefficient between single phase and two-phase regions. This study applies a computational fluid dynamics (CFD) study on the two phase flow and the heat transfer characteristics in the condensing-superheated zone. The condensation process of R410A (Tsat=44.59 °C) in a horizontal pipe (Di=6.1 mm) is simulated by using VOF (volume of fluid) model. The formation and the development of the condensation film are detailed analyzed. The heat transfer mechanism in the condensing-superheated zone includes both the single-phase convection and the phase transition. The heat transfer coefficient in the superheated condensation area increases from the single-phase convection to the saturated condensation. Combined with the development of liquid film morphology, this study is helpful for understanding the flow and heat transfer mechanism in the condensing-superheated zone.

The paper proposes a model of heat transfer in the evaporator of the spacecraft thermal control system. The model allows to calculate the average temperature of the evaporator wall and to build a "boiling curve" in a wide range of thermal loads. Adequacy of the model is confirmed by experimental studies on an aluminum thermal sink with high longitudinal thermal conductivity in the range of parameters typical for the thermal control systems of spacecrafts. Ammonia is used as a working fluid. The model might be recommended for use in zero gravity and normal ground conditions.

The paper presents results of experimental research on the influence of the mass flux on the heat transfer during flow boiling of subcooled de-ionized water in conditions of three-side heating in a system of two microchannels with an aspect ratio of 13.3. The horizontal experimental section was a copper block with two microchannels 2 mm wide, 0.15 mm deep, and 16 mm long; the inner surface of the microchannels was covered with a thin layer of nickel with hydrophilic nanoparticles of Aerosil 200. The mass flux varied from 500 to 3500 kg/m extsuperscript2s; the initial water subcooling relative to the saturation temperature at the microchannel outlet was 80°C. The dependence of the superheat of the wall on the heat flux density was determined; the critical heat fluxes were measured; comparison of the obtained data with calculation based on the binomial equation for subcooled boiling in a microchannel was carried out.

The present article experimentally explored heat transfer through a horizontal metal foam tube with a constant wall heat flux during the subcooled flow boiling of water, Al2O3/H2O, and CuO/H2O nanofluids. To investigate the effect of using metallic foam on the subcooled flow boiling heat transfer, the results have been compared with a simple tube. Besides, The impacts of significant parameters, including subcooled inlet temperature (25–60 °C), mass flux (150–310 kg m−2 s−1), wall heat flux (100–230 kW m−2), particle type (CuO and Al2O3), and nanofluids concentration (0.5% wt., 1% wt.) were investigated on the transfer of boiling heat while measuring pressure loss and rate of heat transfer. The results show that the use of metal foam increases the heat transfer rate by 3.5–5.8 times compared to the simple tube. Comparing the subcooled flow boiling heat transfer of water and nanofluids in both metal foam and simple tubes, it was found that water has superiority over the nanofluids. Besides, the value of the thermal performance index defined as a ratio of the heat transfer enhancement to the pressure loss degradation is more than one for water and the nanofluids.

Growth of a nonisothermal bubble on a heated horizontal surface in a subcooled flow is studied to determine the significance of different heat transfer mechanisms on the bubble growth. The heat tra...

Condensation of vapor bubbles in a subcooled liquid is known to influence heat transfer and pressure oscillation in subcooled boiling and direct contact condensation. This study reviews the published literature concerning interfacial heat transfer and bubble dynamics in the process of bubble condensation. The correlations for bubble condensation are analyzed and evaluated with a database covering a wide range of Reynolds, Jacob, and Prandtl numbers. Then, the investigations addressing bubble dynamics are reviewed, which focus on the bubble condensation patterns, motion, collapse, and the pressure oscillations induced by bubble condensation, as well as the effect of noncondensable gas and field. Despite the extensive experiments of bubble condensation available in the literature, it is shown that there is still a shortage of investigation focused on the variation of thermal boundary layer and turbulence formed near the bubble at the micro-scale, which could help to develop the prediction method of bubble condensation in the future. The transportation of noncondensable gas inside the mixture bubble and effect of capillary waves formed on the bubble surface on the actual vapor-liquid contact area and thermal boundary are also suggested to be further investigated to gain the thorough understanding of the bubble condensation process.

The present study elucidates the design and experimentation of a minichannel evaporator in an R134a vapour compression refrigeration system for electronics cooling applications. In the current study, a calculation module was developed to design a minichannel evaporator to keep the surface temperature of the chip below a certain value for reliable operation conditions in electronic cooling applications. In the calculation module, the conventional-scale heat transfer correlation was used to predict the surface temperature of the chip. On the other hand, the conventional-scale and microscale pressure drop correlations were tested to assess the pressure drop in the minichannel evaporator. The proposed calculation module was verified using experimental tests for different heat loads. It was found that the proposed calculation model predicted very well the experimental data of the surface temperature of the chip for all heat input. The calculation module with micro-scale pressure drop correlation predicted well the experimental pressure drop data in the minichannel evaporator for all heat loads. Moreover, the effects of the degree of subcooling, superheating degree and condensation temperature on the surface temperature of the chip and pressure drop in the minichannel evaporator were investigated to determine optimum operating conditions at different cooling capacities using the calculation module. The results showed that the increase in the degree of subcooling enhances the performance of the minichannel evaporator. On the other hand, the lower degree of superheating and condensation temperature yielded better performance for the minichannel evaporator. The feasibility of the results for electronic cooling applications is discussed based on the findings.

In this Paper, the lattice Boltzmann method (LBM) is adopted to simulate bubble dynamics and heat transfer characteristics along inclined structured surfaces in subcooled boiling. First, the growth...

In this study, the heat transfer characteristics of one-side heated screw tubes were analyzed using subcooled flow boiling experiments. The experiments were performed until the critical heat flux was reached, while the applied heat flux was gradually increased. Subsequently, the boiling curves derived from the experimental results were divided into partial nucleate boiling (PDB) and fully developed nucleate boiling (FDB) regimes. As the flow rate increases, the forced convective heat transfer is enhanced, improving the single-phase heat transfer performance. However, as the FDB region is approached, most of the heat transfer proceeds via nucleate boiling heat transfer, so the heat transfer coefficient curve converges into the same straight line. The same trend was observed when analyzing the effect of the degree of subcooling. However, as the system pressure increased, and the liquid surface tension and latent heat decreased, a faster transition was observed from the single-phase regime to the PDB regime. From evaluating the prediction performance of the existing subcooled flow boiling heat transfer correlations, among the PDB regime; Liu and Winterton correlation, a kind of Chen-type correlation, showed the highest accuracy with mean absolute error (MAE) of 13.62% and root mean square error (RMSE) of 16.13%. However, most of the FDB correlations did not accurately predict the heat transfer performance of the one-side heated screw tube under a high heat flux load of several MW/m2. Therefore, a new screw tube FDB correlation was developed using Python code combined with artificial intelligence technology. The authors believe that the proposed correlation will be useful for not only predicting the heat transfer performance in the FDB region, but also establishing an operating map of the screw tube in the future.

A clear understanding of bubble dynamics and heat transfer characteristics of hydrogen boiling in microgravity is significant for achieving safe and high-efficiency utilization of liquid hydrogen in space. In the present paper, a numerical simulation model is developed to predict the subcooled pool film boiling for liquid hydrogen in different gravities. The computations are based on the volume of fluid method combined with Lee's phase change model. The results show that the bubble released from the wavy gas-liquid interface might grow to a larger size before departure with the decrease of gravity, and poor heat transfer performance is observed in reduced gravity. However, once the gravity level is low enough or the subcooling of liquid is sufficiently large, instead of bubble formation and release at the vapor-liquid interface, a thin gas film layer is almost observed and maintained in the surface of horizontal flat or wire heater.

In nuclear power plants, steam generators are important parts. This paper introduces a U-tube steam generator (UTSG) model implemented completely using MATLAB environment. The UTSG consists of four regions: upward and downward primary regions, upward and downward metal tube regions, and secondary regions, which contain heat transfer region, steam separation region, and subcooled water region. Governing equations are derived by applying energy and mass conservation equations in all regions. Accurate functions that describe the relationships between thermodynamic properties of the saturated steam are introduced instead of interpolation method that is widely used. Steady state and one transient case are presented as well.

In the context of central solar tower systems, tubular receivers are among the most appealing absorber solutions: the absorbed solar radiation is transferred from the tube external surface to the heat transfer fluid (HTF) flowing within the absorber. In the case of air as HTF, very high temperatures of the coolant can be obtained in principle, thus increasing the efficiency of the downstream thermodynamic cycle. To explore the possible applicability of a porous medium made of Raschig Rings (RRs), already successfully adopted in the heat removal from the resonant cavity of a technological device, the gyrotron, where the heat flux can go up to 20–25 MW/m2 and removed by subcooled water, a mock-up of a planar receiver equipped with RRs has been tested in a solar furnace, using air as coolant. The test results are presented here and analyzed 1 . Furthermore, a numerical model of the mock-up, where the RRs are modeled in detail by the Discrete Element Method, is presented and its capability to reproduce the measured data demonstrated. The model shows, for the tested configuration, an enhancement of the heat transfer of a factor of ~5 with respect to a plain channel with the same envelope, and a Performance Evaluation Criteria of 2–2.5 when the device is compared to the same receiver configuration, but without RRs.

A closed loop two-phase thermosiphon is supposed as an effective heat transfer unit for improving the heat transfer in the latent heat storage devices, which has a simple structure and natural circulation. An experimental study and theoretical analysis were conducted to evaluate the heat transfer performance. The effects of height difference, filling ratio, and condenser temperature were investigated relative to changes in pressure and evaporator temperature. The results indicate that heat transfer performance is improved with an increase in height difference and a decrease in condenser temperature. Meanwhile, a closed loop thermosiphon with medium filling ratios of 44.5% and 49.9% shows the best heat transfer performance. Furthermore, the subcooling phenomenon occurs at large filling ratios of 68.1% and 67.1% and the superheating phenomenon occurs at small filling ratios of 27.3% and 31.1%.