The main objective of this work is to develop a numerical model to analyze heat transfer and condensation of a rising spherical bubble. The model included the bubble shrinkage during condensation, which can be utilized to analyze the bubble’s total energy loss, raising velocity, and condensation rate of a single bubble compared to multiple bubbles with the same total thermal energy. The equations of motion, heat, and mass transfer were developed. The model results were verified with the bubble condensation experiment data in the literature, in which they exhibited a good agreement. For the validation, the model results were compared with bubble condensation experiment data in the literature, which showed a good agreement with the experimental results. The dynamic term of the model is developed using the force balance on a gravity-driven bubble, including hydrodynamic drag force and gravity/buoyancy force, which acting with and against the bubble’s motion direction. For the thermal part of the model, a condensation correlation has been adapted to represent the Nusselt number as a function of Reynolds number (Re), Jakob number (Ja), and Prandtl number (Pr). A MATLAB code is developed in order to calculate the instantaneous velocity, the radius, and the mass loss of the vapor bubble in each time step. Moreover, the fundamental behavior for a single bubble and multiple bubbles was investigated in various initial conditions under the same total thermal energy. The effects of the initial bubble radius and the temperature difference between the liquid and vapor phases were analyzed for both scenarios in order to examine the condensation rate. It was found that the thermal behavior of the condensing bubble can be improved by forcing the bubble to collapse into sub bubbles, which will increase the total interfacial area and the rising velocity. Farther, due to generated sub bubbles, the resultant velocity increased the turbulency and the heat transfer rate accordingly. This study can lead to improve the heat transfer rate and allow for more intensive research to enhance the condensation rate.

Cyclone venturi dryer is suitable for drying materials with large particle size and wide distribution. The working process of cyclone venturi dryer is a very complicated three-dimensional and turbulent motion, so it is difficult to be studied theoretically and experimentally. In order to study the internal flow characteristics of the biomass particle cyclone venturi dryer, the computational fluid dynamics (CFD) software was used to simulate the gas-solid two-phase flow field inside the cyclone venturi dryer. The continuous phase adopts the Realizable k-ε turbulence model and the particle phase is discrete. The effects of different injection volume on the pressure, velocity, and temperature fields inside a cyclone venturi dryer were analyzed. The results showed that the maximum pressure drop and velocity change inside the dryer were at the venturi pipe. The wet material of the cyclone venturi dryer was inhaled into the venturi contraction tube by the negative pressure formed after the highspeed airflow was ejected, thus the mixture was completed in the venturi throat. The wood debris material was mixed with the high-speed hot gas flow in the venturi throat and then sprayed into the diffusion pipe. In the diffusion pipe of venturi, the heat and mass transfer process of wet wood debris and heat flow in venturi diffusion tube was completed. It is in good agreement with the simulation results. This study can provide a reference for the optimization design of the related cyclone venturi dryer structure.

The liquid metal high-temperature oscillating heat pipe (LMHOHP) is a kind of high efficiency heat transfer device, which can function in high-temperature environments above 500°C. In this paper, the effects of inclination angle on the startup and heat transfer performance of a LMHOHP were investigated experimentally. The sodium-potassium alloy (potassium 78%) was used as the working fluid of the LMHOHP and the filling ratio was 50%. The start-up characteristics and heat transfer performance of the LMHOHP at four inclination angles of 0°, 30°, 60° and 90° were tested when the operating temperatures were 150°C and 400°C, respectively. Experimental results show that (1) The LMHOHP can start-up and function at all the tested inclination angles, the maximum temperatures of the evaporator and condenser can exceed 1000°C and 700°C, respectively. (2) The thermal resistance of the LMHOHP decreases with inclination angle increases, the thermal resistance at the inclination angle of 90° decreases by up to 32.9%, 41.6% and 55.9% compared with that at the inclination angle of 60°, 30° and 0°, respectively. (3) When the input power exceeds 3000W, the flow patterns of LMHOHP at the inclination angle of 90°, 60° and 30° can be changed from the oscillating motion to the unidirectional circulating flow. (4) Compared with the operating temperature of 150°C, the heat transfer performance of the LMHOHP improves at the operating temperature of 400°C, at the input power of 3457W and the inclination angle of 90°, the minimum thermal resistance of LMHOHP is 0.075°C/W. The results shown that the LMHOHP has a good adaptability to working conditions which further extends the application range of oscillating heat pipe.

Additive manufacturing (AM) allows for the fabrication of complex metallic specimens which are otherwise very difficult or impossible to create using traditional manufacturing methods. For multi-phase heat spreaders such as vapor chambers, complex internal geometry lends itself well to AM as new design features can be realized. The internal geometries of vapor chambers frequently involve several thin pillars, which act as wicking structures and support a small internal cavity in which the phase change of a liquid, usually water, provides a means to effectively ‘spread’ a high heat flux throughout a larger area. In this case study, Laser Powder Bed Fusion (L-PBF) AM was utilized to create a series of aluminum alloy (AlSi10Mg) support pillars of fixed height with varying filet angles and build orientations (0°, 10°, 20°, 30°, 40°, 50° and 60° from the normal surface) to determine effects on surface roughness and resolution. Surface-to-water contact angles were measured on rinsed, un-polished surfaces, and microscopy was employed to obtain images demonstrating surface roughness profiles. This was done on both heated and unheated surfaces. Results clearly demonstrate that the surface roughness of upward and downward-facing surfaces during L-PBF depends on build orientation. The observational findings of this experiment showed that the surface roughness on surfaces representing horizontally oriented faces decreased as the printing orientation increased. This indicates that one must be cognizant of printing orientation for the L-PBF of capillary structures. A change in surface roughness as a result of printing orientation effects the heat transfer within an enclosed space due to a change in contact angles between the liquid and the heated surface. The results of this study will aid in future design considerations of heat pipes for their additive manufacture via current L-PBF technology.

Microchannel heat sinks have attracted much attention due to its superiority of removing high heat flux with a very small size, which recently have been applied in the electronic cooling. The flow boiling of deionized water in the ultrahigh-aspect-ratio copper microchannels was experimentally investigated in this work. The heat sink consists of 20 rectangular straight microchannels (5000μm × 200μm) with the hydraulic diameter of 385 μm and the aspect ratio of 25. The experiment was conducted with the heat flux range of 39.7–1368 kW/m 2 and the mass flow range of 21.3–41.6 kg/(m 2 ·s) (the inlet temperature of 30°C). The flow patterns in the ultrahigh-aspect-ratio microchannels were captured using the visualization technique and four flow regimes (bubbly flow, slug flow, churn flow and annual flow) were clearly observed. The nucleate boiling sites increase significantly in the depth direction on the large-area side wall at bubbly flow. An obvious delay for flow patterns transformation in the upper regions compared with the lower region of microchannel was found. The bubble sliding was observed with a large sliding distance compared to the conventional low-aspect-ratio microchannels, which leads to an enhanced the heat transfer. It shows that the higher effective heat flux and the lower pressure drop were achieved in ultrahighaspect-ratio microchannel due to its special flow patterns in the channel and large surface area in the limited volume.

A comprehensive three-dimensional numerical model is developed to evaluate the effect of bending on water-copper cylindrical heat pipes. This model distinguishes itself from other models by its ability to uniquely determine the operating pressure of the heat pipe based on the operating and physical conditions. The effects of one 90-degree bend and two 90-degree bends are evaluated on the performance of a heat pipe. Two types of wicks are considered: a screen mesh wick and a sintered powder wick. The obtained results show that bending does affect the vapor pressure drop; however, the changes are not significant when compared to the operating pressure of the heat pipe. If the bending is performed in a manner where the wick is not damaged and the liquid is not blocked from returning to the evaporator, the performance of the heat pipe will not be affected significantly. In addition, if the heat pipe is operating in the horizontal direction, where both evaporator and condenser legs are at the same level, bending does not affect the liquid pressure drop significantly; however, the screen mesh does provide a higher capillary limit. The results also showed that the effects of gravity can be important when bending heat pipes and consideration should be given for this factor. When the bent heat pipe works against gravity, the sintered powder wick heat pipes showed higher capillary limits.

This study investigates the cooling performance of a passive cooling system for electric motor cooling applications. The metal-based phase change materials are used for cooling the motor and preventing its temperature rise. As compared to oil-based phase change materials, these materials have a higher melting point and thermal conductivity. The flow field and transient heat conduction are simulated using the finite volume method. The accuracy of numerical values obtained from the simulation of the phase change materials is validated. The sensitivity of the numerical results to the number of computational elements and time step value is assessed. The main goal of adopting the phase change material based passive cooling system is to maintain the operational motor temperature in the allowed range for applications with high and repetitive peak power demands such as electric vehicles by using phase change materials in cooling channels twisted around the motor. Moreover, this study investigates the effect of the phase change material container arrangement on the cooling performance of the under study cooling system.

Wettability has been proved as an important issue to the thermal transport at solid-liquid interface at different scales, however, its enhancement mechanism has not been clearly understood till now. In this study, the nucleate boiling behavior of argon fluid on heterogeneous wetting surfaces were examined with the non-equilibrium molecular dynamics (MD) method, the ring-patterned and stripe-patterned schemes were designed and analyzed, respectively. By comparing the boiling inception time and evaporation rate of liquid argon atoms, it is found that the ring-patterned surface shows an advantage in the nucleate boiling heat transfer compared with the stripe-patterned one. The differences in heat transfer characteristics for different surfaces can be explained through the qualitative analysis of fluid density distribution and solid-fluid interaction energy. Furthermore, the boiling phenomena on ring-patterned surfaces with alternated hydrophilic and hydrophobic intervals were simulated to study the influence of area fraction of hydrophilic region on the heat transfer performance. It is observed that bubble nucleus firstly appears over the hydrophobic region of the substrate. The substrate with more hydrophilic area will have a better heat transfer performance. It is also demonstrated that there is an optimal area fraction, which can make the evaporation rate of fluid reach the highest value. The findings in this work can contribute to the design and fabrication of nanocoating surface to enhance its heat transfer performance under high heat flux condition.

In this work, we study the heat transfer performance and particle dynamics of a highly mass loaded, compressible, particle-laden flow in a horizontally-oriented pipe using an Eulerian-Eulerian (two-fluid) computational model. An attendant experimental configuration [1] provides the basis for the study. Specifically, a 17 bar co-flow of nitrogen gas and copper powder are modeled with inlet Reynolds numbers of 3 × 10 4 , 4.5 × 10 4 , and 6 × 10 4 and mass loadings of 0, 0.5, and 1.0. Eight binned particle sizes were modeled to represent the known powder properties. Significant settling of all particle groups are observed leading to asymmetric temperature distributions. Wall and core flow temperature distributions are observed to agree well with measurements. In high Reynolds number cases, the predictions of the multiphase computational model were satisfactorily aligned with the experimental results. Low Reynolds number model predictions were not as consistent with the experimental measurements.

Accurate measurement of fuel quality is critical for automotive applications as it impacts engine performance and emissions. A number of techniques have been proposed to measure fuel quality including acoustic wave speed sensors, chemometric modeling, near-infrared spectrophotometry, Raman spectroscopy etc. All of these techniques are complex in nature and require expensive equipment. In contrast, we propose a novel, simple and rapid method for estimating fuel quality in field environments. This involves measuring the evaporation time of fuel droplets in the Leidenfrost state, and using the results of statistical data analysis conducted on an experimental data bank comprising evaporation time data for similar droplets. The Leidenfrost state refers to a liquid droplet hovering on its own vapor layer on a superheated surface. To showcase our approach, evaporation time was measured for droplets consisting of isopropyl alcohol (IPA) and water blends with varying parameters such as IPA fraction (0-1), droplet volume (20–100μL) and surface temperature (200–340 °C). The resulting data bank (96 data points) was used to train and evaluate the performance of a polynomial regression (using a semilogarithmic transformation) model in predicting the evaporation time as a function of the above-mentioned parameters. R 2 accuracies of 97.34% (training data), 96.82% (test data), 97.46% (total) and a relative error within ± 0.25% for the entire dataset was obtained using the regression model, which highlights the predictive capabilities of our approach.

This paper was removed from publication at the author’s request. October 2, 2020. Copyright © 2020 by ASME

A flow boiling heat transfer experimental investigation on the shell-side of a smooth tube and two enhanced tubes (C-EHT, P-EHT) was performed using R410A as working fluid. The outer surface of the C-EHT tube is covered with special fins having a salient height, and the inner surface has a thread structure with the same distribution density. The outer surface of the P-EHT tube has lots of tiny pits formed by high-strength sandblasting and smooth inner surface. The inlet and outlet vapor quality is 0.2 and 0.8 at a saturation temperature of 279K. The flow boiling experimental results show that the evaporation heat transfer coefficient of C-EHT tube is the largest among the tested tubes. However, for the P-EHT tube, its heat transfer coefficient is lower than that of smooth tube. C-EHT tube with special fins has higher heat transfer area, which can also intensify the interfacial turbulence with increasing fins. The reason why the flow boiling heat transfer coefficient of the P-EHT tube is lower than that of the smooth tube is that the diameter of the pit on P-EHT tube is too small to form the nucleation cites, which results in bubble retention under the condition of low mass flux and deteriorates the flow boiling heat transfer performance.

The boiling behaviour on the surface of large diameter tubes is known to be strongly dependent on the local orientation of the surface around the circumference. Understanding such local variations in boiling behaviour is of particular interest in the CANDU ® nuclear industry, where part of the heat removal path for cooling the fuel under postulated accident conditions is via pool-boiling on the surface of the 132 mm diameter calandria tube that is immersed in a pool of the heavy water moderator. While the average pool boiling behaviour of the calandria tubes has been well studied with integrated experiments, local boiling correlations have not been developed for prototypical diameter calandria tubes. Local boiling correlations will allow for more detailed modelling of postulated accident scenarios and better quantification of safety margins. In this study, the nucleate and film pool-boiling characteristics of a large diameter Zircaloy-2 CANDU ® calandria tube were tested in a pool of subcooled water. A novel technique is developed to derive the local boiling curve along the circumference of the tube, involving the local heating, measurement of the wall temperature, and heat flux. The adequacy of the technique is determined by comparing the local boiling curves with previous experiments in which averaged pool boiling characteristics were obtained.

Micro/nanostructured wicking surfaces with different submicron-scale substructures were fabricated by using electrochemical deposition and chemical etching methods on stainless steel spheres. Quenching experiments were carried out on these surfaces in saturated water to reveal the effect of submicron-scale substructures on the boiling heat transfer enhanced by wickability. The results indicated that, as compared to the knife-like submicron-scale substructure, the needle-like substructure improved the wickability of wicking surface, but has a weaker boiling heat transfer enhancement. The difference in wicking/spreading performance between two wicking surfaces with different submicron-scale substructures could not be enough to significantly affect the boiling heat transfer during quenching. However, the knife-like substructure might have a larger specific surface area, resulting in a greater boiling heat transfer enhancement. Besides, liquid spreading distance measured at room temperature might not be suitable for the wickability characterization.

Steam condensation heat transfer was studied over a honeycomb-like microporous superhydrophobic surface under various pressures, in order to elucidate the effects of pressure on the jumping-droplet condensation behaviors. The condensing pressure was varied from 4 kPa to 13 kPa, based on the typical operating conditions of condensers in power plants. Stable coalescence-induced droplet jumping was realized on the honeycomb-like superhydrophobic surface over this range of pressure, leading to a great enhancement on the condensation heat transfer as compared to that on the common hydrophobic surface, especially at small degrees of subcooling (e.g., < 10 K). The frequency and number of jumping droplets were observed to decrease at lower pressures because of the less amount of condensate produced, and at higher degrees of subcooling due to the occurrence of surface flooding. The increasing condensing pressure was found to lead to a later onset of surface flooding. The results indicated that the honeycomb-like superhydrophobic surface has a great potential for industrial condensation equipment operating at multiple pressures owing to its superior performance and facile fabrication.

Advanced manufacturing techniques have improved dramatically in recent years and design freedom for engineered components and systems has never been greater. Despite these advancements, the majority of our design tools for thermal-fluids systems are still rooted within traditional architectures and manufacturing techniques. In particular, the complex nature of two-phase flow and heat transfer has made the development of design methods that can accommodate these complex geometries enabled by new manufacturing techniques challenging. Here, we investigate a new design method for two-phase flow systems. We conduct a multiobjective parameter study considering two-phase flow and heat transfer through a single channel with a circular cross section. To increase our design degrees of freedom, we allow the channel to increase or decrease in cross-sectional area along its flow length, but constrain the channel inlet and outlet to a constant hydraulic diameter. Maximizing heat transfer and minimizing pressure drop are the two design objectives, which we evaluate using two-phase heat transfer correlations and the Homogeneous Equilibrium Model. We find that using small expansion angles can greatly reduce two-phase flow pressure drop and also provide high heat transfer coefficients when compared to straight channel designs. We present a set of feasible designs for varying input heat fluxes, liquid mass flow rates, and channel orientation angles and show how the ideal expansion channel angle varies with these operational conditions.

When condensation first forms on a surface, it starts as tiny droplets. As the surface continues to collect condensation, the droplets grow together and form a film. The film increases the thermal resistance of the system. It is possible to remove the fluid from the condensing surface before it develops into a film. Dropwise condensation has the capability of providing up to an order of magnitude higher heat transfer than film condensation. A hydrophobic surface is capable of sustaining dropwise condensation but creates a high energy barrier that restricts nucleation. A hydrophilic surface has a low energy barrier for nucleation but retains the water quickly transitioning to film condensation. A hydrophilic and hydrophobic patterned surface creates a surface with a low nucleation energy barrier and is capable of sustaining dropwise condensation. Surface patterns are evaluated under laminar flow conditions to maximize mass collection. The surfaces are evaluated using a thermal model, which includes an equivalent thermal resistance for diffusion. Laminar flow rates are evaluated using Reynolds numbers from 1,218 to 4 × 10 5 . Hydrophilic nodules sizes are evaluated from 0.1 mm to 3.7 mm. Under natural convection flow, mass collection can be increased by 20% with respect to film heat transfer.

This work conducts a multiphase multi-physics numerical study on this mixed mode cooling system, where evaporation of a liquid water film augments the convective cooling by evaporative heat and mass transfer. The mathematical framework consists of coupled mass, momentum, energy equation with species transport of air and evaporated water in the gas phase. Author’s prior works on perfectly flat surface predicted a minimum 500% increase of heat transfer coefficient in presence of a dual mode heat transfer. Current effort conducts a parametric study based on the different surface roughness structures over a broad range of Reynolds number condition. Array of circular, squared and triangular shaped grooves have been introduced along the surface exposed to the evaporating liquid. The cooling performances have been analyzed for different surface roughness structures and then compared against the flat film configuration. Furthermore, pressure drop across the liquid-gas interface has also been analyzed to predict the pumping power of the system. Model predicts that triangular shaped grooves exhibits a maximum ∼22% decrease in thermal resistance with a minimum pressure drop (∼5.5 times) across the interface. The square shaped surface morphology achieves 18% reduction in thermal resistance, with a penalty of pressure drop increase by a factor of ∼8. In comparison, circular grooves exhibit similar thermal performance but with a slightly a lower pressure drop (∼7 times). The numerical predictions have been compared with similar experimental findings and qualitative agreement was observed.

Single-phase and flow condensation experiments were performed using refrigerant R410A in the outer annular region of horizontal enhanced tube with different enhanced surfaces at a saturation temperature of 45°C in the range of mass flux 44.43–102.23kg/(m 2 s). The vapor quality ranges from 0.8 to 0.2. The outer diameters of the tubes are all 19.05mm, but the inner diameters are slightly different due to different surface structures. The surface structures of the three enhanced tubes are fins(EHT1 tube), toothed structures (EHT2 tube) and fine cavities(EHT3 tube) of different sizes and densities. Among them, the EHT3 tube has internal threads. Wilson diagram method was used to determine the enhancement ratio of the water side heat transfer coefficient of EHT3 tube. It was found that the pressure drop increased with the increase of mass flux, while the heat transfer coefficient showed different trends, and the smooth tube was always the lowest of the four tubes. A comprehensive evaluation factor α combining heat transfer enhancement factor ( EF ) and pressure drop penalty factor ( PF ) was defined, in which EHT2 tube (1.38–1.75) was the largest, with strong heat transfer capacity and small pressure drop, so the condensing heat transfer capacity of EHT2 tube was the best.

An investigation into the conditional stability of direct contact steam condensation is presented. Direct contact steam condensation systems heat process fluids more effectively than indirect contact heat exchangers because the total energy of the steam is introduced directly into the process fluid, bypassing any resistances associated with having to cross the heat exchanger boundary. In the present work, superheated steam is injected perpendicular to a flow of subcooled liquid water. Conditional stability of direct contact steam condensation stems from the type of fluctuations exhibited by the steam plume structure. Under certain flow conditions the steam plume condenses in the form of a stable conical jet with little to no acoustic noise or resultant pressure fluctuations. However, other conditions cause the steam plume to increase in oscillation intensity, eventually transitioning to an intermittent bubbling structure that results in high-amplitude acoustic noise and large pressure fluctuations in the flow of liquid water. A regime map is presented to establish the process criteria for the onset of instability of condensation. The transitional profile to instability is also presented as an analysis of plume oscillation frequencies and associated pressure oscillations propagating from these fluctuations. It is found that, as the mode of condensation goes unstable, steam plume oscillations increase in intensity and decrease in frequency until the structure collapses into bubbles that provoke strong pressure oscillations in the surrounding water.