Abstract In this study, both electro-dynamic balance (EDB) method and pendant droplet method were employed to study the evaporation and crystallization process of brine droplet. The EDB setup was used to levitate the charged micrometer sized droplets to study the evaporation process. The pendant droplet method could hang millimeter sized droplet to study the crystallization process. The evaporation of brine droplets with different mass concentrations was visualized by a high speed camera under different temperatures and relative humidity in the test chamber. The results showed that EDB method could get the accurate droplet evaporation results which obeyed the classic D2-law. It was found that the evaporation was increased with the decrease of relative humidity. Due to the attractive force provided by sodium and chloride ion on water molecules, evaporation rate of brine droplet was smaller than pure water, therefore the evaporation rate decreased with the increase of mass concentration by showing a linear relationship. In addition, a salt shell could be formed at the outside of droplet which still contained some amount of salt solution inside. Consequently, the water molecules need to overcome the pressure difference inside and outside the salt shell, and diffuse through the shell for further evaporation. For a higher relative humidity, a more round shell would be formed during the evaporation, and the growth of crystallization along the filament is weaker than that of smaller relative humidity. We hope this study can provide a different perspective to the heat transfer community about the evaporation of brine droplet.

Abstract The food-energy-water nexus considers critical resource challenges which must be resolved in order to meet the needs of a growing population. Agriculture is the largest global water user, accounting for two-thirds of global water withdrawals, including water for crop irrigation. Understanding and therefore reducing evaporation of water from soil is an approach to conserve water resources globally. This work studies evaporation of water from a simulated soil column and employs x-ray imaging to determine the location of water in the porous media. A 30-mL beaker was filled with approximately 1700 2-mm hydrophilic glass beads. Water (i.e., 5.5 mL) was added to the simulated soil, comprised of glass beads and a heat flux (i.e., 1500 W/m 2 ) was applied to the beaker using a solar simulator and the intensity was measured with a light meter. Real-time mass measurements were recorded during evaporation and X-ray imaging was utilized to capture liquid transport during evaporation. Images were post-processed using Matlab; the position of the liquid front was determined from this imaging. Across three replications, it took 47 hours on average to evaporate 5 mL of the total 5.5 mL of water. The transitions between evaporation Stage I, II, and III evaporation rates were determined using mass data and x-ray imaging; transition between Stages I and II occurred between approximately 4 and 9 hours, and the transition from Stage II to III evaporation occurred between approximately 18 and 24 hours. The result of this experiment will be useful to understand the liquid transport and formation of liquid bridges during evaporation from soil.

Abstract Due to increasing power densities and decreasing device footprints, thermal management has become an important design requirement in modern electronic devices. Loop heat pipes are phase change-based devices that can absorb and transport large heat fluxes via the latent heat of evaporation of a working fluid. However, these devices are bulky and difficult to miniaturize due to the constraining effect of undesired parasitic heat flow and other thermodynamic considerations of the two-phase flow loop. Here, we present experimental results demonstrating the operation of an ultra-thin microscale loop heat pipe that employs a planar evaporator wick designed to counter the negative effects of parasitic heat flow. Despite the extremely low wick thickness (< 0.5 mm), the device is able to successfully startup, with no apparent observation of a wick dry-out due to parasitic heat flow-induced disruptions of liquid supply to the evaporator. A latent heat flux of approximately 6.7 W/cm 2 is absorbed per unit area of the evaporator during the device startup phase.

Abstract In this work, heat transfer coefficient during condensation of a refrigerant on the outside surface of a copper tube with wavy fins was experimentally investigated. To fully characterize the condensation heat transfer, the experiments were conducted under two conditions: no refrigerant overfeed and subject to various degree inundation. The results under the condition of no overfeed are compared with the Beatty and Katz model. While the trend of degradation with increasing subcooling was in good agreement with the model (within 5%), the condensation heat transfer coefficients from the wavy fins were 11–15% higher. Based on the Nusselt model, the surface tension effect is not taken into account in the Beatty and Katz model, which plays an important role in condensation on a surface with fins. The photographs taken during the experiments showed that the condensate dripping columns have a pitch is in agreement with that proposed by Yung et al. [24] for falling film evaporation applications. The second part of the experiments under the various degree of inundation provides further insight into the heat transfer capability of the surface with wavy fins.

Abstract Pool boiling and in-tube condensation phenomena have been investigated intensively during the past decades, due to the superior heat transfer capacity of the phase change process. In passive heat removal heat exchangers of nuclear power plants, the two phase-change phenomena usually occur simultaneously on both sides of the tube wall to achieve the maximum heat transfer efficiency. However, the studies on the effects of in-tube condensation on external pool boiling heat transfer are very limited, especially in numerical computation aspect. In the present study, the saturated pooling boiling over a vertical tube under the influences of in-tube steam condensation is investigated numerically. The Volume of Fluid (VOF) interface tracking method is employed based on the 2D axisymmetric Euler-Euler multiphase frame. The phase change model combining with a mathematical smoothing algorithm and a temporal relaxation procedure has been implemented into CFD platform by user defined functions (UDFs). The two-phase flow pattern and bubble behavior have been analyzed. The effects of inlet steam mass flow rate on boiling heat transfer are discussed.

Abstract This work aims to study the effects of the net-type spacer on the performance of direct contact membrane distillation (DCMD) modules. Laminar and k-ω SST turbulence models are used to conduct simulations in three-dimensional modules with and without spacers. The spacers are placed in the middle of the feed and permeate channel. The net type spacers of diameter 0.25 h and 0.5 h were considered, where h is the height of each channel. The inlet temperature of the feed and the permeate channel set to 353 K and 293 K . The feed Reynolds number is varied (500, 1500) while the permeate Reynolds number is fixed at 330. We revealed that the presence of spacer in the flow channels mitigates both the temperature and the concentration polarization and yields higher vapor permeation. We also showed that the module containing larger size spacers yields better flux performance and lower level of temperature and concentration polarization. Moreover, the modules containing spacers become more efficient as the feed flow rate is increased.

Abstract Computational fluid dynamics simulations are conducted to study the performance of the sweeping gas membrane distillation module (SGMD) for seawater desalination process. The main objective of this work is to study the effect of membrane properties on the membrane flux performance and temperature and concentration polarization characteristics of the module. CFD simulations are conducted in a three-dimensional module to characterize the steady-state velocity, temperature and concentration field in the feed and permeate channel. The Reynolds number for the feed and the permeate stream are set to 900 and 2000, and thus the laminar flow model is adapted for each channel. The effects of the porosity and the membrane thickness are varied while the pore size is fixed for the parametric study. It is revealed that the membrane thickness has a profound influence while the membrane porosity has a slight influence on the SGMD performance. We observed a high level of temperature polarization within the module, which adversely affects the system performance. Remedies for mitigating temperature polarization should be considered for future studies.

Abstract Diatoms are a group of single-celled photosynthetic algae that use biochemical pathways to bio-mineralize and self-assemble three-dimensional photonic crystals with unique photonic and micro- & nano-fluidic properties. In recent years, diatom biosilica has been used in surface-enhanced Raman scattering (SERS) based optofluidic sensors for detection of a variety of chemical and biological molecules. In this paper, we present a study to develop a microfluidic pumping system using super-hydrophilic diatom thin films. The desire to develop such a system stems from the requirement to create a low-cost, self-powered microfluidic pumping system that can sustain a continuous flow over an extended period of time. The diatom biosilica acts not only as the driving force behind the flow, but also serves as ultra-sensitive SERS substrates that allows for trace detection of various molecules. Liquid is drawn from a reservoir to the tip of a 150μm inner diameter capillary tube positioned directly over the diatom film. A thin and long horizontal reservoir is used to prevent flooding on the diatom film when the liquid is initially drawn to the diatom film through a capillary tube from the reservoir. The connection of the meniscus from the capillary to the film was maintained from a horizontal reservoir for a recorded time of 20 hours and 32 minutes before the partially filled reservoir emptied. Flow rates of 0.38, 0.22 and 0.16μL/min were achieved for square biosilica thin films of 49mm 2 , 25mm 2 , and 9mm 2 at a temperature of 63°F and 45% relative humidity respectively. A temperature-controlled system was introduced for the 49mm 2 substrate and flow rates of 0.60, 0.82, 0.93, and 1.15μL/min were observed at 72, 77, 86, and 95°F at 21% relative humidity respectively. More testing and analysis will be performed to test the operation limits of the proposed self-powered microfluidic system.

Abstract The refrigerant retained on heat transfer surfaces has a deleterious impact on the performance of heating, ventilation, air conditioning and refrigeration systems, which not only increases the thermal resistance between the vapor and surface, but also requires a higher charge to the system. In this work, a new paraffin coating has been applied on condensation surfaces, and R134a condensate retention has been studied on both copper plate and fins with (without) coating. The heat transfer coefficient was measured based on the one-dimensional heat conduction method and the retention was quantified using image processing. The results show that the heat transfer has been enhanced on the coated surfaces under a wide range of subcool degree, with a maximum increase of 27.4% in heat transfer coefficient; a reduced liquid retention has also been observed on paraffin coated fins with the retention area ratio decreased by 35.1% to 47.1% (depending on different subcool) compared to the uncoated fins. This work shows great potentials for reducing retained liquid and enhance heat transfer during refrigerant condensation.

Abstract Continuous power supply in Concentrated Solar Power (CSP) plants can be achieved via integration of efficient, cost-effective and reliable Thermal Energy Storage (TES) system. The new generation of CSPs operates at higher temperatures and requires thermal storage systems with higher energy density at high storage temperature. Thermochemical Energy Storage (TCES) is the available solution which can meet performance requirements of energy density, temperature, and stability. TCES systems apply reversible endothermic/exothermic chemical reaction through which energy is stored as the enthalpy of reaction and released during the reverse mode. Among several available potential reversible chemical reactions, metal oxides, with high reaction temperature and enthalpy of reaction, have remarkable advantages compared to others. They use air both as Heat Transfer Fluid (HTF) and oxidation reactant, which eliminates the need for storage and intermediate heat exchanger integration between HTF and collector working fluid. Using air as HTF has made them perfectly fitted for the new generation of air operated solar collectors. Among several screened available potential metal oxides, cobalt and manganese oxides were selected as best candidates for high-temperature storage. Pure manganese oxide does not meet the cyclic operation requirement, but the iron-doped solid solution has proven reasonable cyclic storage performance. In this study, iron-doped manganese oxide (Fe-Mn 1:3 molar ratio) has been selected as a redox agent for TCES reactor. The cylindrical packed bed configuration is considered as a reactor bed configuration. A two-dimensional axisymmetric numerical model is developed using the finite element method. Performance analysis for both charge and discharge is provided separately. The effect of inflow rate and bed porosity variations on reactor performance in complete storage cycle were studied.

Abstract The performance of a falling-film heat exchanger is strongly linked to the surface characteristics and the heat transfer processes that take place over the tubes. The primary aim of this numerical study is to characterize the influence of surface wettability of tubes on the falling film flow mode and its associated surface heat transfer. Surface wettability is generally characterized by the contact angle and, in this study, the wettability characteristics ranged from superhydrophilic to a superhydrophobic tube surface. The dynamic motion of the triple contact line connecting the solid, liquid and gas phases over the tube surface is replicated with the help of the Kistler’s dynamic contact angle model. The volume of Fluid (VOF) simulations was carried out for the flow and heat transfer of liquid flow over horizontal tubes with different surface wettabilities. The mass flow rate is such that the flow is in the jet mode where the liquid flows in the form of jets in between the horizontal tubes. This corresponds to a liquid mass flow rate per unit tube length of 0.06 and 0.18 Kg/m-s, under which the inline and staggered jets modes of flow are observed. Under the two flow rates and different surface wettabilities, the liquid flow hydrodynamics over the tube surfaces was explored in terms of the liquid film thickness, the contact areas (solid-liquid and liquid-air) between the different phases, and the heat transfer coefficient. The axial resistance imposed by the increasing contact angle tends to inhibit the extent of the liquid spreading over the tube surface and this, in turn, influences the liquid film thickness and the wetted area of the tube surface. A significant decrement in the heat transfer rate from the tube surfaces was observed as the equilibrium contact angle ranged from 2° to 175°. Heat transfer characteristics were quantified over a wide range of contact angles for the two mass flow rates. Fluid recirculations were observed in the liquid bulk which had a major impact on the heat transfer distribution over the tube surface.

Abstract In this paper a numerical investigation on mixed convection in confined slot jets impinging on a porous media is accomplished. The working fluids are pure water or Al2O3/water based nanofluids and a single-phase model approach has been adopted in order to describe their behavior. A two-dimensional configuration is analized and different Peclet numbers and Rayleigh numbers are considered. The thermal non-equilibrium energy condition is assumed to execute two-dimensional simulations on the system. The examined foams are characterized by distinct values of pores per inch, PPI, equal to 5, 10, 20 and 40. The particle volume concentrations range from 0% to 4% and the particle diameter is equal to 30 nm. The target surface is heated by a constant temperature value, calculated according to the value of Rayleigh number. The distance of the target surface is five times greater than the slot jet width. The aim consists into study the thermal and fluid-dynamic behaviour of the system. Results show increasing values of the convective heat transfer coefficients for increasing values of Peclet number and nanoparticle concentration. Furthermore, the heat transfer coefficient presents a different behavior at varying PPI numbers for different Peclet numbers.

Abstract Deep understanding of nucleate boiling heat transfer mechanism of saline solution is of great importance for the design and safe operation of steam generation equipment. In this paper, the nucleate flow boiling process of saline solution in a vertical heated pipe was experimentally studied within the concentration range of 0 % ∼ 6 %. In order to realize the visualization, the vertical heated pipe was made of transparent silica glass and a transparent ITO heater was used to provide energy for boiling. The high-speed high-resolution camera was used to capture the vapor-liquid two-phase flow structure. The bubble behaviors such as bubble departure diameter, bubble departure frequency, bubble growth time and waiting time were investigated under different operating conditions. The experimental results showed that the heat transfer deterioration did not occur within the solution concentration of 6% in this work. Under some low heat flux conditions, the heat transfer coefficients of solution can be higher than those of pure water. The reason for this phenomenon can be explained by the different bubble behaviors. Comparing to pure water, the bubble departure diameter of saline solution is bigger and bubble departure frequency is lower. The influences of operating parameters, including concentration, mass flux (200 kg/m 2 s ∼ 600 kg/m 2 s), heat flux (30 kW/m 2 ∼ 180 kW/m 2 ) and subcooling of fluid (5 K ∼ 35 K), on the nucleate boiling heat transfer coefficients and bubble parameters were comprehensively studied.

Abstract Additive Manufacturing (AM) particularly laser powder-bed fusion, is advancing rapidly in manufacturing industries. Selective laser melting (SLM) also known as 3D-printing has become one of the most recent developed and extensively used techniques for several manufacturing processes. However, processing parameters influence the defect formation mechanisms such as porosities, holes, cracks, incomplete fusion and molten pool configuration during the SLM process of metallic powders. Even though an extensive amount of work have been done in minimizing these defect formations by simply varying processing parameters such as laser power, deposition thickness and scanning speed, it is of great importance to study the heat transfer mechanisms in laser heating process and utilize the optimum process parameters in minimizing the residual stress and strain as well as improving the quality of a manufactured product. In this present work, the authors implement a numerical thermo-mechanical model approach to determine the residual stress and strain in 316L Stainless Steel built samples using a finite element method (FEM) software ANSYS ® (Workbench version 19.0). We are able to predict the unsteady temperature distribution of temperature-dependent thermal properties, residual stress and strain as a result of the rapid melting and solidification of 316L Stainless Steel metallic powder with optimized processing parameters. From the simulation result, it is shown that the residual stress decreases with an increase in scanning speed, hatch distance and preheat temperature. However, an increase in melting temperature also increases the residual stress and strain in the simulated 3D built part.

Abstract In this research, the transient heat transfer due to exponentially increasing heat input was experimentally measured for upward water flowing in a vertical small tube. The heat generation rate was increased exponentially with a function of Qoexp ( t /τ), where, Qo is an initial heat generation rate, t represents time and τ is e-folding time. The heat generation rate was controlled by high speed computer system. The test tube was heated with exponentially increasing heat input by direct current. The average temperature of test tube was measured by resistance thermometry using a double bridge circuit. The experimental apparatus consists of a test section, a cooler, a heater, a pump, a tank and a pressurizer. The working fluid was distilled and deionized water. The inlet fluid temperature of test tube was controlled by the cooler and the heater. The system pressure was up to 800 kPa. The test tube was 0.7 mm in inner diameter and 12.0 mm in heated length respectively. The ratio of heated length to inner diameter was 17.1. The test tube was electrically isolated from experimental loop by Bakelite plates. The experimental data were compared with previous correlations of nucleate boiling. It was obtained that the experimented data agree well with full-developed flow boiling correlation by Rohsenow. Moreover, the transient critical heat flux (CHF) and nucleate boiling with onset of nucleate boiling (ONB) values increased with the increase in flow velocity. The transient CHFs and ONBs increased with a decrease in e-folding time at τ < 1 s, and they approached steady-state value at τ > 1 s. It was understood that the heat transfer is in steady-state at τ > 1 s, and it is in transient state at τ < 1 s.

Abstract We study the evolution of the solid-liquid interface during melting and solidification of a material with constant internal heat generation and prescribed heat flux at the boundary for a plane wall and a cylinder. The equations are solved by splitting them into transient and steady-state components and then using separation of variables. This results in an ordinary differential equation for the interface that involves infinite series. The initial value problem is solved numerically, and solutions are compared to the previously published quasi-static solutions. We show that when the internal heat generation and the heat flux at the boundary are close in value to each other, the motion of the phase change front takes longer to reach steady-state than when the values are farther apart. As the difference between the internal heat generation and the heat flux increases, the transient solutions become more dominant and the numerical solution of the phase change front does not reach steady-state before the outer boundary or centerline is reached. The difference between the internal heat generation and the heat flux at the boundary can be used to control the motion and speed of the interface. The problem has applications for a nuclear fuel rod during meltdown.

A numerical investigation of a single highly confined bubble moving through a millimeter-scale channel in the absence of phase change is presented. The simulation includes thermal boundary conditions designed to match those of completed experiments. The channel is horizontal with a uniform-heat-generation upper wall and an adiabatic lower boundary condition. The use of a Lagrangian framework allows for the simulation of a channel of arbitrary length using a limited computational domain. The liquid phase is a low-Reynolds-number unsteady laminar flow, and the phase interactions are modeled using the Volume-of-Fluid method with full geometric reconstruction of the liquid/gas interface. Results are presented for two bubble sizes, two liquid flow rates, and two Prandtl numbers. The paper focuses on heat transfer in the rearward wake of the bubble. Nusselt numbers for the higher Prandtl number case are shown to follow a power law relationship with distance behind the bubble. Important dynamical structures include a pair of vortical structures at the rear of the bubble associated with cold fluid being brought near the wall and fluid jets oriented in the transverse direction to either side of the bubble.

Experimental condensation heat transfer data from our previous studies were collected to find a suitable heat transfer correlation for three-dimensional surface enhanced heat transfer tubes. Dimples/protrusions and petal arrays make up the enhanced surface of the 1EHT tube, while longitudinal grooves and dimples constitute that of the other two 2EHT tubes. Working fluids investigated includes three refrigerants R22, R32, R410A and three enhanced tubes have a same nominal inner diameter of 12.7mm. Due to the unique and complex surface structure of three EHT tubes, a constant is proposed and utilized for each EHT tube to estimate the impact of enhanced surfaces on interfacial turbulence, boundary layer disruption, flow separation and secondary flow generation. In addition, the Cavallini correlation modified with the constant can predict almost all the data points within a ±5% error band, which can be a good example for how to predetermine the heat transfer coefficient of three-dimensional enhanced tubes with a unique surface.

In this study, a combination of synchronized high-speed video (HSV) and infrared (IR) thermography was used to characterize the nucleation, growth and detachment of bubbles generated during nucleate boiling inside the nanoemulsion fluid. The Ethanol/Polyalphaolefin nanoemulsion fluid was formed by dispersing ethanol nanodroplets into base fluid Polyalphaolefin, in which these nanodroplets can serve as the pre-seed boiling nuclei. With this unique combination, it allows controlled nucleation, time-resolved temperature distribution data for the boiling surface and direct visualization of the bubble cycle to track bubble nucleation and growth. Data gathered included measurements of bubble growth versus time, as well as 2D temperature history of the heater surface underneath the bubbles. Our findings demonstrate a significant difference of bubble dynamics between the nanoemulsion fluid and pure ethanol, which may also account for the substantial increase in heat transfer coefficient and critical heat flux of nanoemulsion fluid. It is also observed here that the bubbles occurred inside the nanoemulsion fluid appear to be more uniform and two orders-of-magnitude larger in size. While the growth rate of the bubbles inside pure ethanol was found to be heat diffusion controlled at a coefficient around ½, which however, dropped to be around 0.3 for nanoemulsion fluid. Further study on this unique system will help reveal its heat transfer mechanisms.

An experimental study on local flow boiling heat transfer performance in two longitudinal dimple-grooved tubes and an equivalent smooth tube was performed. All three test cooper tubes have the same inner diameter of 11.5 mm; the working fluid is the near-azeotropic mixture, R410A; and all test runs are conducted in a 2 m long horizontal tube-in-tube heat exchanger. Constant evaporation temperature at 10 °C was maintained when heat flux ranging from 32 kW/m 2 to 37 kW/m 2 and refrigerant quality varied from inlet 0.1 to outlet 0.9 at mass flux 150 kg/(m 2 s). The local heat transfer coefficients were obtained for all test conditions using refrigerant R410A. The test results for evaporation were presented compared to the equivalent smooth tube. Wall temperature and local and average heat flux is measured; heat flux effect and surface superheat effect is discussed on the tube side evaporation. The enhanced heat transfer area of two longitudinal dimple-grooved tubes are 1.02 and 1.03, respectively.