Swapnil Dubey

Splashing of high speed droplet train impinging on a hot surface

Transition phenomena are observed when a water droplet train (with up to 18.9 m/s in velocity and 39.20 kHz in frequency) impinges onto a heated copper surface (up to 250 °C). The hydrodynamic flow pattern strongly depends on the wall temperature. The surface temperature does not apparently influence the spreading speed when the wall temperature is less than the boiling temperature, but it enhances the spreading rate significantly at higher surface temperatures. A “steady-state” wetting surface area can be reached when the water supply rate equals the water consumption rate. The time-independent spreading diameter decreases with an increase in the wall temperature until an ultimate diameter of the “steady-state” wetting area is observed at around 0.4 mm, 3.4 times the droplet diameter, when the surface temperature is higher than 190 °C. Moreover, unlike the random direction splashing when wall temperature is less than 180 °C, a stable splashing angle is established at higher wall temperatures. However, th...

Impact of the relative humidity on the LNG cold energy based inlet air cooled microturbine systems

Vaporization of liquefied natural gas (LNG) can be used as heat sinks in the external thermal cycles, and these processes are known as LNG cold utilization systems. Inlet air cooling application is one of the LNG cold utilization methods in the power generation sector, and it provides lower compressor inlet temperatures to increase the thermal efficiency and the net generated work. This study performs the simulations of energetic, environmental and economic approaches for the 30 kW and 65 kW microturbine systems for various relative humidity values under different ambient air temperatures. It is found that the relative humidity did not have a crucial impact on the net generate work rate and thermal efficiency although it causes a slight decrement from 60% RH to 90% RH. The environmental analyses show that the emission rate increases by the rising of relative humidity. Moreover, the environmental payback period increases by the rising of the relative humidity value. Lastly, the payback periods are investigated and it is seen that the relative humidity and the ambient air temperature do not have any crucial impact on the payback periods. The payback periods are found as 4.34 and 3.27 years for the 30 kW and 65 kW models, respectively.

The transitions of time-independent spreading diameter and splashing angle when a droplet train impinging onto a hot surface

The hydrodynamic patterns of the impingement of a water droplet train on a high temperature substrate are captured with a high-speed camera, and then analyzed. The tested droplets range from 8.8 m s−1 to 18.9 m s−1 in velocity, from 92 μm to 121 μm in diameter and from 28.01 kHz to 56.56 kHz in frequency. Three different orientations of the impact droplet trains are tested. It is found that the hydrodynamic pattern varies significantly with the wall temperature. The time-independent spreading diameter as well as the stable splashing angle reduces with the increase in wall temperature. The two transitions exist in all the experiments with various droplet impact velocities, frequencies or impact angles. Once the splashing is established, the size, the velocity magnitude and the moving direction of splashed secondary droplets obey a bell-shaped distribution. A lower impact velocity renders a wider range of the secondary droplet sizes. The lowest impact velocity case presents outstanding splashing characteristics in the post-transition regime, indicating that an impact velocity of around 10 m s−1 would be a threshold. Those two transitions are not notably influenced by the droplet frequency but significantly affected by the impact angle. The transition of the splashing angle is observed at a lower wall temperature when the droplet train is inclined.

Two-Dimensional Numerical Analysis of Membrane-Based Heat and Mass Cross-Flow Exchanger

ABSTRACT A two-dimensional numerical simulation model for a membrane-based heat and mass exchanger was developed. The system model equations were used to determine the coupled heat and moisture transfer from the humid air to the high concentrated liquid desiccant solution (LiCl, lithium chloride) by means of a parallel stack hydrophobic permeable membrane. The two streams of air and liquid desiccant solution were arranged in cross-flow directions. The fourth-order Runge–Kutta method was employed to solve these system model equations in a steady-state condition. This model enables one to predict the latent effectiveness of a membrane-based parallel cross-flow exchanger for dehumidification purpose in response to air to liquid mass flow ratio and the mass transfer unit number.

Point heat sink induced by droplet train impingement

A point heat sink is produced by impinging a high frequency microscale droplet stream onto a superheated copper substrate. Although the overall target surface area is larger than the liquid-solid interface by two or three orders of magnitude, the thermal energy is mainly removed through the point heat sink rather than the rest dry area. Therefore, the spherical conduction patterns in the solid materials are observed with a “nozzle-shifting” method which requires only two temperature probes. The temperature gradient in the vicinity of the impingement stagnation point is tremendously high, suggesting that the liquid-solid interface temperature is significantly lower than the far-field bulk temperature of the substrate. Moreover, the liquid-to-solid heat transfer is measured, which agrees well with the theoretical prediction. The maximum interface heat flux can reach around 80 W/mm2. It is insensitive to the substrate temperature in a relatively wide temperature range, which brings conveniences to the potent...

Droplet train impinging onto a solid substrate surface

The heat transfer characteristics are experimentally investigated when a droplet train impinges onto a heated surface. A steady-state experimental method is applied to measure the heat transfer coefficient continuously. The boiling regime, transition of the splashing regime, and post-transition regime can be observed. The hydrodynamic pattern significantly changes with an increase of wall temperature. The associated heat transfer characteristics are diverse in different regimes. In the boiling regime, the heat flux increases with an increase of wall temperature. A peak value is reached when the splashing is just established. However, when it steps into the transition regime, the wall heat flux reduces with an increase in wall temperature. At the end of the transition regime, a sudden drop of the heat flux is found. In the post-transition regime, the heat flux increases with an increase of wall temperature again, whereas the heat transfer coefficient is kept a constant.

Low-grade heat collection from a latent heat thermal energy storage unit

Thermal energy storage is widely employed in waste heat recovery. Phase change materials (PCMs) are a type of promising thermal storage (TES) approach featuring high energy stored or released during melting and freezing. In this study, a latent heat storage system is designed to collect the waste heat from hot air while its efficiency is experimentally investigated. The system consists of five cuboid chambers to store PCMs and six air channels to deliver the exhausted gas. Paraffin wax is selected as the tested PCMs and differential scanning calorimetry (DSC) is applied to calibrate its melting range and latent heat. In the experiments, the effects of average air velocities and inlet air temperature on the performance of the latent heat storage unit are experimentally investigated. The experiments show that the designed TES unit can store maximally thermal energy for near 4510 kJ within 250 min, while the latent heat occupies 15 % at fan speed 25 RPM and the heater power with 2 kW. It is indicated that increasing of air mass flow rate and increasing of air inlet temperature can improve the heat transfer performance. It is found that the temperature profiles in the freezing process are smoother than those in the melting process.

The Impingement of Droplet Train Onto a Flat Hot Surface With High Wall Superheat

Single droplet based investigations have been performed for hundreds of years. However, in many industrial applications, such as printing, spray cooling and coating etc, numerous droplets will be produced. Droplet train, therefore, is a physical model to approach the complex situation. When the wall temperature is higher than the boiling point, the problem becomes even complex. The subcooling of the droplet, the superheat of the wall also influence the hydrodynamic pattern of the droplet impingement. The hydrodynamic behavior of the water droplet train impinging onto a hot surface (up to 220 °C) is investigated. A droplet train generator is employed to produce stable high velocity (around 6.35 to 19.13 m/s) droplet train (with a diameter around 0.1 mm) at the droplet frequency ranges from 27990 Hz to 55560 Hz. The hot surface is made by copper and heated with cartridge heaters. The effect of wall superheat on flow pattern is experimentally examined and reported. The results show that the wall temperature plays an significant role to the impingement. It influences the spreading speed, stable spreading diameter and splashing angle apparently.Copyright