Biomass oil has attracted extensive attention due to its carbon neutrality, high energy density, and renewable nature. During its spray combustion, the droplet impacting heated wall surfaces is of paramount significance. Despite extensive research, few studies focused on the splashing behavior of a mixing oil droplet impacting a thin film on a heated wall, particularly on transition regimes and characteristics of secondary droplets. The present work experimentally studied the splashing behavior of a mixing gasoline-camellia oil (GCO) droplet impacting its thin film on heated walls, with a focus on the secondary droplets and their energy ratio. Their kinematic features (detachment time, velocity, splashing angle) were characterized by analyzing the effects of Weber number (We), Ohnesorge number (Oh) and the wall temperature Tw. Their energy ratio was further estimated through the statistical analysis of their counts and diameters. Novel concise correlations for the detachment time and the energy ratio of the secondary droplet were respectively developed by considering We and Oh. The results show that the detachment time is primarily governed by Tw and Oh, with a minimal influence of We. Splashing angle typically ranges from 30 degrees to 50 degrees, while splashing velocity increases with Tw and We. The count of secondary droplets, while influenced by Oh values, increases with We, gradually converging to a constant value for increasing Tw. The energy ratio of the total secondary droplets exhibits a parabolic behavior as a function of the product of We and Oh. The results demonstrate that a 50 % mixture of the camellia oil-gasoline still exhibits good splashing behavior (secondary atomization), while the optimal mixing ratio is about 25 %. These founding get valuable insights into the heat transfer mechanism involved in the impact, gasification and combustion of GCO.

Heat exchangers operate under high temperatures, pressures, and corrosive environments, which can lead to mechanical failure of metal pipes in energy and process plants. Double-walled pipes have been proposed to mitigate these mechanical failure risks in extreme operating conditions. The thermal contact conductance (TCC) at the interface plays a crucial role in determining the performance of double-walled pipes. This study employed a phase-sensitive transient thermal method to investigate the effects of surface roughness and operating temperature on thermal performance in shrink-fitted 316 L stainless steel double-walled pipes. Surface roughness was systematically controlled using abrasive flow machining (AFM) before pipe assembly. Experimental results revealed a clear correlation between surface finish and thermal performance: TCC increased from (5.40 f 1.03) x 103 W/m2K to (13.70 f 1.80) x 103 W/m2K as surface roughness decreased from 1.95 mu m to 0.06 mu m at a constant interface pressure of 23.91 MPa. Beyond 0.06 mu m, further roughness reduction provided no significant improvement, indicating an optimal threshold for heat transfer maximization. Temperature effects were also examined. For the double-walled pipes assembled at room temperature, the thermal contact conductance (TCC) dropped sharply from (18.37 f 0.53) x 103 W/m2K to (4.01 f 0.90) x 103 W/m2K when heated to 500 degrees C. This decline was attributed to thermal expansion reducing residual interfacial stress. To address this issue, double-walled pipes were assembled at 400 degrees C. This approach maintained stable thermal performance, with TCC varying only slightly from (17.81 f 0.16) x 103 W/m2K to (15.42 f 0.64) x 103 W/m2K under 500 degrees C operating conditions. These results demonstrate that assembly at 400 degrees C effectively prevents thermal degradation, even under extreme temperatures.

Liquid film thickness is a dominant feature for understanding boiling heat transfer mechanism in microscale slug flow. Flow boiling in circular microchannels has been extensively studied. Microchannels with non-circular cross-section are more common in industrial applications, but there have been few studies on such complex cross-sections. In the present study, the transient liquid film thickness during flow boiling in non-circular microchannels was experimentally investigated by a laser confocal displacement meter. Non-circular tubes with inner dimension of 0.39 x 0.39, 0.5 x 0.5, 0.6 x 0.6, 0.7 x 0.7 and 0.3 x 0.8 mm2 2 were used for the test section, and water and ethanol were used as working fluids. The variation of liquid film thickness under adiabatic condition in non-circular microchannels was analyzed and an empirical correlation was proposed for predicting initial liquid film thickness. On this basis, a new theoretical model for liquid film thickness variation under flow boiling in non-circular microchannels was developed, considering the effects of evaporation, shear force and transversal flow.

Micro liquid film plays an important role during flow boiling in microchannels. Current studies on flow boiling in microchannels have mainly focused on circular microchannels. However, microchannels with sharp corners, such as square and rectangular microchannels, are more common in practice. In the present study, flow boiling experiments were performed in square microchannels using ethanol as the working fluid. The mass flux was 26.71 kg (m- 2 s- 1) and heat flux ranged from 10 to 80 kW m- 2. The transient liquid film thickness during flow boiling in square microchannels was measured using a laser confocal displacement meter. The pressure drop between inlet and outlet of the microchannel and the wall temperature were measured synchronously. Experimental results demonstrated that heat transfer performance depended on liquid film thickness and the thickness in square microchannels was smaller than in circular microchannels under the same capillary number. The variation of liquid film thickness during flow boiling in square microchannels was analyzed and a theoretical model was proposed for predicting liquid film thickness. According to variation in wall temperature and liquid film thickness, the flow boiling process in square microchannels was divided into liquid slug, elongated bubble, and dryout zones. The heat transfer mechanism of different zones was analyzed. In addition, the microchannel pressure drop fluctuation and backflow phenomenon were analyzed.

Slug flow is a common flow regime in microchannel flow boiling cooling devices. The thickness of the liquid film around the bubble determines the heat transfer efficiency. Because of the existence of bubbles, the pressure drop of slug flow is different from that of single-phase flow. In this work, the two-phase flow with single gas slug in a 1-mm-diameter circular microchannel is numerically studied. Water and glycerol-water solution are used as the working liquid and air is used as the working gas. The results indicate that the bubble shape is stable at low velocity, while the bubble tail is wavy at high velocity. If the velocity increases further, the bubble will break. For slug flow at the same velocity, the film thickness of gas slug in glycerol-water solution/air condition is larger than that in water/air condition. The wall resistance of slug flow is mainly located at the bubble nose and tail, while the resistance in the middle of the bubble can almost be ignored.

The present work focuses mainly on the effects of heat input, filling ratio, inclination angle, tube diameter and coolant temperature on the thermal performance of a wraparound heat pipe charged with R134a. Results show that thermal resistance decreases with the increase of heat input when the filling ratio is larger than 40%. An optimal filling ratio for the heat pipe with the best performance exists between 50% and 60%. The pressure of working fluid in the heat pipe exceeds 1.6 MPa in the 70% and 80% filling ratios experiments. For larger inclination angles (theta > 10 degrees), the thermal resistance decreases with increasing the heat input and finally tends to a stable value. For heat loads of 420 W and greater, the values of thermal resistance are 0.056, 0.07, 0.034 and 0.027 K/W for outer diameters of 8, 10, 12 and 16 mm, respectively. No significant difference in thermal resistance at different coolant temperatures is observed for heat inputs greater than 300 W. In all experiments, for a 22 degrees inclination angle, an outer diameter of 16 mm, and a filling ratio of 50%, the best performance of heat pipe is observed and the lowest value of thermal resistance is 0.027 K/W.

The three-phase contact line area plays a vital role in the heat dissipation of micro-devices due to its intense heat transfer capacity. However, the scale of the three-phase contact line area is small, and experimental studies are mainly used to observe the film profile. Few experiments can characterize this area with high spatial resolution, making it a well-known challenge to make a quantitative assessment of the limit of heat transfer capacity. In this work, we use the time-domain thermoreflectance system to measure the liquid film heat transfer capacity in the contact line region. The effective thermal conductivity of the liquid film obtained from the experiment can well reflect the heat transfer capacity of the liquid film at different positions and can be used to calculate the evaporating heat flux and the overall heat transfer coefficient of the liquid film. We also established a theoretical model and performed numerical calculations compared with the experimental results. The results show that the liquid film has the most vital heat transfer capacity in the evaporating thin-film region. The overall heat transfer coefficient can reach-439 kW/(m(2).K), and the evaporating heat flux of the liquid film can reach -1.8 x 10(6) W/m(2). This work provides a new idea for the experimental study of the three-phase contact line area, offers experimental support for theoretical research, and lays a foundation for revealing the heat and mass transport mechanism in the three-phase contact line area. (C) 2022 Elsevier Ltd. All rights reserved.

In this paper, three working fluids of deionized water, anhydrous ethanol and FC-72 were used to study the transient variation of liquid film thickness on the bubble's periphery for the flow boiling in micro-channels. The experiments showed that the liquid film declined rapidly under the coupled action of evaporation and shear force of vapor flow until it was dried out. A transition point existed on the curvature of liquid film thickness, and the liquid film thickness at the transition point (initial liquid film thickness) could be predicted by the Taylor flow, which was similar to the liquid film on the bubble's periphery in the adiabatic two-phase flow. Comparisons of the three working fluids showed that the reduction speed of the liquid film thickness depended on many thermophysical properties, including surface tension, liquid viscosity and latent heat. The heat flux also had evident influences on the liquid film behavior. An empirical model was proposed by considering evaporation of the liquid film and shear action on the vapor-liquid interface. The present correlation predicts the transient variation of film thickness within +/- 15% error and is applicable for all working fluids. (C) 2020 Elsevier Ltd. All rights reserved.

Most boiling heat transfer experimental correlations assume that the formation and growth of bubbles at adjacent nucleation sites are independent, but experimental results show that interactions do occur between adjacent nucleation sites, while, the experimental results have some divergence because of the complexity of boiling. In this paper, we conducted a numerical study by lattice Boltzmann method on the nucleation site interactions during pool boiling. The LBM is featured of a multiple-relaxationtime algorithm with hybrid thermal scheme which can carefully control the parameters that influence boiling and keep other factors unchanged. The numerical results show that temperature at the given nucleation sites has strong dependence at short separation distance, and consequently the wall superheat could change the thermal and hydrodynamic interactions between nucleation sites. Moreover, wettability also plays a critical role in boiling, so the effects of wettability on nucleation site interactions were also studied, results show that nucleation site interactions on hydrophobic surface are mainly promotive, which is different from hydrophilic surface. These findings can be used to better understand the mechanism of nucleation site interactions and explain the divergence of previous experimental results. (C) 2019 Elsevier Ltd. All rights reserved.

More and more fixed geometry structures are being manufactured to enhance the boiling heat transfer (BHT). However, they usually perform well at a special heat load and don't always have good BHT properties. Applying shape memory alloy (SMA) material to change the geometry is a new solution to achieve optimal effect at different boiling condition. Pool boiling heat transfer on deformable structures made of SMA in three fluids (ethanol, FC-72, water) with different thermal properties was explored. Comparing heat flux versus wall superheat and heat transfer coefficient (HTC) at different fluxes with fixed geometry, it was found that deformable structure combines the merits of closed-tunnel and open-tunnel. At low heat fluxes, it can increase the number of nucleation sites inside the closed tunnels with bent fins and after reverting to the original shape, the nucleation sites are activated and the bubble growth and departure is accelerated to enhance the HTC significantly. So by choosing the appropriate time and opportunity for different fluids to open the tunnels, the deformable structures can be used to achieve adaptive-control of boiling heat transfer. In terms of theoretical analysis, the existing correlations for fixed structures have not been fit for deformable structures, because large-scale deformation make the heat transfer mechanism different from the fixed geometry structure. Thus experimental results are compared with fitting curves and a new correlation was deduced. (C) 2017 Elsevier Ltd. All rights reserved.

A number of technologies have been developed to enhance boiling heat transfer (BHT). The enhancements of BHT depend on the size and geometry of the micro/mini structures and it seems difficult to design a structure that is optimum for all heat transfer conditions. This letter reports a study on adaptive control and enhancement of BHT by shape-memory alloy (SMA) structures. The experimental results of BHT on structured porous surfaces show that the SMA surface with recoverable structures has advantages for heat transfer both in the improvement of heat transfer coefficient and in the extending of operating range. The potential applications of such enhancement structures in diverse heat transfer devices are perhaps the most exciting.

The effects of vertical mechanical vibration on the heat characteristics of liquid film in vertical rectangular microgrooves are observed. The vibration frequencies are 6Hz, 10Hz and 30Hz, respectively; the vibration amplitudes are in the range of 1.95∼3.23mm. Three sizes of rectangular microgrooved plate are used in experiments. The microgrooved plate is vertically mounted on a vibration plane; DC heat load is added on the back wall of the microgrooved plate. Vibration of the liquid film in the microgroove is observed by a high-speed digital camera, and temperature on the back of the plate is recorded by a data acquisition. The experimental results show that temperature on the plate back decreases obviously with the increase of the vibration frequency or amplitude, heat transfer of the microgrooved plate is intensively enhanced. The main reason is that the forced convections on the groove surface and in the liquid film, caused by the mechanical vibration, enhance the heat transfer. The investigation provides more information for the application of the micro-configuration heat sink under fierce vibration conditions.

A micro particle image velocimetry (micro-PIV) system was used to investigate the shape of the meniscus and the flow characteristics in open rectangular microgrooves heat sinks. In experiment, the rectangular microgrooves heat sinks were placed in horizon, the inclined angle of 15, 25 and 35°. The results show that the shape of meniscus in rectangular microgrooves heat sinks in horizon, the inclined angle of 15, 25 and 35° with input heat flux is parabolic. At low heat flux, the shape of meniscus in horizontal microgrooves heat sink could be regarded unchanged and never touches bottom of the microgrooves; in inclined microgrooves, the bottom of the shape of meniscus gradually moves downward to the microgroove bottom. Flow stages in rectangular microgrooves heat sinks verify the assumption that there exist continuous accommodation stage and corner flow stage. The mean curvature of meniscus in horizontal microgrooves almost does not change along axial direction of microgrooves. The mean curvature of the meniscus increases along the axial direction of inclined microgrooves heat sinks. Velocity decreases from the middle to both of the side walls. The mean velocity profile is not symmetrical, and this is due to the different roughness of two side walls.

Deformation of the triple-phase contact line in various sizes of rectangular microgrooves under vertical vibration conditions was studied in this paper. Width of the rectangular microgroove ranges from 0.2 mm to 0.4mm and depth of the microgrooves is 0.2∼0.6mm. The frequency of vibration is 10Hz, and the amplitude of vibration is approximately 3.5mm. The research results show that oscillation of the liquid film in microgrooves becomes more obvious, and the triple-phase contact line is deformed more greatly when the groove width or the groove depth increases. The main reason is that the flow resistance of the liquid film in microgrooves decreases when the groove width or the groove depth increases.

The mathematical model is established in this article to describe the relationship between the wetting length of working liquid in the triangular wetting region of rectangular capillary microgrooves and the geometric dimension, tilt angle, type of working liquid and heat flux when heating the back of microgrooves heat sink. The model supposes that the vapour-liquid interface of meniscus is quadratic parabola but not arc. The predictions from the theoretical analysis are successfully compared with the experimental results.

Evaporation of liquid meniscus formed in microgrooves is associated with very high heat transfer rates, but the cross-section shape of air-liquid interface has a great influence to the heat transfer in microgrooves. But the real cross-section shape of interface in microgrooves is still unknown for us. In this work, the micro-PIV (Particle Image Velocimetry) method is used to test the cross-section shape of air-liquid interface in microgrooves. In the experiment, the camera is focus on different planes from top of the microgrooves to the bottom of the microgrooves. In each plane, we can see the boundary between the air and liquid through small particles added into the liquid. The positions of boundary in each plane for a given cross section are drawn in two-dimension coordinate. Then the cross-section shape of interface in microgrooves can be seen from the fitting curve. The results show that the cross-section shapes of the interface in microgrooves are not round, but polynomial curves. The curvature of interface in microgrooves changes along a single curve. Besides, the polynomial curves also vary along axial direction of the microgrooves. The variations are more obvious in vertical microgrooves than in horizontal microgrooves.

An infrared thermoviewer is utilized to measure the temperature distribution on solid walls and vapor-liquid interfaces of the rectangular capillary microgrooves heat sink, which is made of borosilicate glass. The infrared thermal image clearly shows that the solid wall temperature of microgroove top is lower than the average temperature of vapor-liquid interface. The results indicate that heat source position has a significant influence on the microgrooves surface temperature distribution, besides working liquid, tilt angle (the angle between microgroove surface and gravity direction) and heat flux.

An important issue in developing applications for photopolymers in holography is the effect of shrinkage on recording properties. In this paper, we introduce a model to describe real-time formation of a single grating in photopolymers at any geometrical angle, under the assumption that the shrinkage is in proportion to the polymerization. This model combines polymerization kinetics with the coupled-wave theory, explaining the shrinkage effect on the diffraction efficiency. The model is validated by comparing its predictions with the experimental results for a film of 99mum thickness.