Mohammad Nasim Hasan

Molecular Dynamics Study on Explosive Boiling of Thin Liquid Argon Film on Nanostructured Surface Under Different Wetting Conditions

Molecular dynamics (MDs) simulations have been performed to investigate the boiling phenomena of thin liquid film adsorbed on a nanostructured solid surface with particular emphasis on the effect of wetting condition of the solid surface. The molecular system consists of liquid and vapor argon and solid platinum wall. The nanostructures which reside on top of the solid wall have shape of rectangular block. The solid–liquid interfacial wettability, in other words whether the solid surface is hydrophilic or hydrophobic, has been altered for different cases to examine its effect on boiling phenomena. The initial configuration of the simulation domain comprises a three-phase system (solid platinum, liquid argon, and vapor argon), which was equilibrated at 90 K. After equilibrium period, the wall temperature was suddenly increased from 90 K to 250 K which is far above the critical point of argon and this initiates rapid or explosive boiling. The spatial and temporal variation of temperature and density as well as the variation of system pressure with respect to time were closely monitored for each case. The heat flux normal to the solid surface was also calculated to illustrate the effectiveness of heat transfer for different cases of wetting conditions of solid surface. The results show that the wetting condition of surface has significant effect on explosive boiling of the thin liquid film. The surface with higher wettability (hydrophilic) provides more favorable conditions for boiling than the low-wetting surface (hydrophobic), and therefore, the liquid argon responds quickly and shifts from liquid to vapor phase faster in the case of hydrophilic surface. The heat transfer rate is also much higher in the case of hydrophilic surface.

Evaporation characteristics of thin film liquid argon in nano-scale confinement: A molecular dynamics study

Molecular dynamics simulation has been carried out to explore the evaporation characteristics of thin liquid argon film in nano-scale confinement. The present study has been conducted to realize the nano-scale physics of simultaneous evaporation and condensation inside a confined space for a three phase system with particular emphasis on the effect of surface wetting conditions. The simulation domain consisted of two parallel platinum plates; one at the top and another at the bottom. The fluid comprised of liquid argon film at the bottom plate and vapor argon in between liquid argon and upper plate of the domain. Considering hydrophilic and hydrophobic nature of top and bottom surfaces, two different cases have been investigated: (i) Case A: Both top and bottom surfaces are hydrophilic, (ii) Case B: both top and bottom surfaces are hydrophobic. For all cases, equilibrium molecular dynamics (EMD) was performed to reach equilibrium state at 90 K. Then the lower wall was set to four different temperatures su...

Atomistic modelling of evaporation and explosive boiling of thin film liquid argon over internally recessed nanostructured surface

Molecular dynamics (MD) simulations have been carried out to investigate evaporation and explosive boiling phenomena of thin film liquid argon on nanostructured solid surface with emphasis on the effect of solid-liquid interfacial wettability. The nanostructured surface considered herein consists of trapezoidal internal recesses of the solid platinum wall. The wetting conditions of the solid surface were assumed such that it covers both the hydrophilic and hydrophobic conditions and hence effect of interfacial wettability on resulting evaporation and boiling phenomena was the main focus of this study. The initial configuration of the simulation domain comprised of a three phase system (solid platinum, liquid argon and vapor argon) on which equilibrium molecular dynamics (EMD) was performed to reach equilibrium state at 90 K. After equilibrium of the three-phase system was established, the wall was set to different temperatures (130 K and 250 K for the case of evaporation and explosive boiling respectively...

Molecular dynamics study on the effect of boundary heating rate on the phase change characteristics of thin film liquid

In this study, theoretical investigation of thin film liquid phase change phenomena under different boundary heating rates has been conducted with the help of molecular dynamics simulation. To do this, the case of argon boiling over a platinum surface has been considered. The study has been conducted to get a better understanding of the nano-scale physics of evaporation/boiling for a three phase system with particular emphasis on the effect of boundary heating rate. The simulation domain consisted of liquid and vapor argon atoms placed over a platinum wall. Initially the whole system was brought to an equilibrium state at 90K with the help of equilibrium molecular dynamics and then the temperature of the bottom wall was increased to a higher temperature (250K/130K) over a finite heating period. Depending on the heating period, the boundary heating rate has been varied in the range of 1600×109 K/s to 8×109 K/s. The variations of argon region temperature, pressure, net evaporation number with respect to tim...

A molecular dynamics study on thin film liquid boiling characteristics under rapid linear boundary heating: Effect of liquid film thickness

This study is a molecular dynamics investigation of phase change phenomena i.e. boiling of thin liquid films subjected to rapid linear heating at the boundary. The purpose of this study is to understand the phase change heat transfer phenomena at nano scale level. In the simulation, a thin film of liquid argon over a platinum surface has been considered. The simulation domain herein is a three-phase system consisting of liquid and vapor argon atoms placed over a platinum wall. Initially the whole system is brought to an equilibrium state at 90 K and then the temperature of the bottom wall is increased to a higher temperature (250K) within a finite time interval. Four different liquid argon film thicknesses have been considered (3 nm, 4 nm, 5 nm and 6 nm) in this study. The boundary heating rate (40×109 K/s) is kept constant in all these cases. Variation in system temperature, pressure, net evaporation number, spatial number density of the argon region with time for different film thickness have been demon...

Thermal transport during thin-film argon evaporation over nanostructured platinum surface: A molecular dynamics study

Investigation of thermal transport characteristics of thin-film liquid evaporation over nanostructured surface has been conducted using molecular dynamics simulation with particular importance on the effects of the nanostructure configuration for different wall–fluid interaction strengths. The nanostructured surface considered herein comprises wall-through rectangular nanoposts placed over a flat wall. Both the substrate and the nanostructure are of platinum while argon is used as the evaporating liquid. Two different wall–fluid interaction strengths have been considered that essentially emulate both hydrophilic and hydrophobic wetting conditions for three different nanostructure configurations. The argon–platinum molecular system is first equilibrated at 90 K and then followed by a sudden increase in the wall temperature at 130 K that induces evaporation of argon laid over it. Comparative effectiveness of heat and mass transfer for different surface wetting conditions has been studied by calculating the wall heat flux and evaporative mass flux. The results obtained in this study show that heat transfer occurs more easily in cases of nanostructured surfaces than in case of flat surface. Difference in behavior of argon molecules during and after the evaporation process, that is, wall adsorption characteristics, has been found to depend on the surface wetting condition as well as on presence and configuration of nanostructure. A thermodynamic approach of energy balance shows reasonable agreement with the present molecular dynamics study.

Numerical study of heat transfer enhancement in an inclined rectangular channel with multiple discrete heaters through active flow modulation

The interaction between the fluid flow in a channel and transversely rotating cylinders across the flow field has a great impact on heat transfer rate. The purpose of this study is to investigate numerically the heat transfer enhancement from multiple discrete heaters placed in an inclined rectangular channel with rotating cylinders inserted above each of the heaters. The whole computation was analyzed for a 2D rectangular channel with upper wall is maintaining a constant low temperature and the bottom wall is provided with multiple discrete isoflux flat heaters connected by adiabatic segments. At inlet, a fully developed parabolic velocity profile at constant low temperature has been induced. Air has been considered as working fluid. The system has been represented mathematically by different sets of governing equations, which are solved by using Galerkine Finite Element method. The effect of the variation of rotating speeds of the cylinders in terms of the cylinder peripheral speed to maximum inflow velocity (ξ), Grashof number (Gr) and channel inclination angle (α) were numerically investigated for the range of 103 ≤ Gr ≤ 106, 0° ≤ α ≤ 90°, 0.5 ≤ ξ ≤ 2.0 at fixed Reynolds number, Re = 100. Temperature field and flow field are investigated in terms of isotherm lines (θ) and stream lines (Ψ) respectively. Heat transfer characteristics are also demonstrated in terms of Normalized Nusselt number (Nu/Nunc) and Local Nusselt number (Nul). The results reveal that insertion of rotating cylinders above heaters increases heat transfer rate from 32% to 55% for all speed ratios (ξ) for the values of Grashof number 103 ≤ Gr ≤ 105 and then start to decrease but still remains about 12% to 22% higher than no cylinder condition at Gr = 106.The interaction between the fluid flow in a channel and transversely rotating cylinders across the flow field has a great impact on heat transfer rate. The purpose of this study is to investigate numerically the heat transfer enhancement from multiple discrete heaters placed in an inclined rectangular channel with rotating cylinders inserted above each of the heaters. The whole computation was analyzed for a 2D rectangular channel with upper wall is maintaining a constant low temperature and the bottom wall is provided with multiple discrete isoflux flat heaters connected by adiabatic segments. At inlet, a fully developed parabolic velocity profile at constant low temperature has been induced. Air has been considered as working fluid. The system has been represented mathematically by different sets of governing equations, which are solved by using Galerkine Finite Element method. The effect of the variation of rotating speeds of the cylinders in terms of the cylinder peripheral speed to maximum inflow vel...

Increasing film cooling effectiveness of a gas turbine blade by using an upstream ramp

In the present investigation, a new design is proposed in order to increase the adiabatic film cooling effectiveness over a symmetric gas turbine blade. Apart from using a row of forward diffused shaped holes for coolant injection, the present study suggests a modification at the upstream of the coolant holes by adding a backward step ramp nearby the hole. Numerical solutions of the Reynolds-Averaged Navier-Stokes and energy equations are obtained by using three-dimensional finite element formulation of Galerkin weighted residual method. Numerical simulation is performed for a row of 25° forward-diffused shaped hole injected at 35° to an advanced gas turbine (AGT) symmetric blade. For parametric investigation, three different ramp heights, h = 0.125D, 0.2D and 0.41D, where D is the diameter of the coolant hole at inlet, along with the blowing ratios, M = 0.3, 0.5, 0.9 and 1.3 are considered, while the turbulence intensity is kept constant at 0.5%. For different combinations of blowing ratio and ramp height, enhanced cooling effects are observed in terms of both local and averaged adiabatic film effectiveness (AFE). Notably this enhancement is of higher magnitude for a higher M-h combination. Additionally, diffusion of the coolant increases in lateral direction in the proposed model.In the present investigation, a new design is proposed in order to increase the adiabatic film cooling effectiveness over a symmetric gas turbine blade. Apart from using a row of forward diffused shaped holes for coolant injection, the present study suggests a modification at the upstream of the coolant holes by adding a backward step ramp nearby the hole. Numerical solutions of the Reynolds-Averaged Navier-Stokes and energy equations are obtained by using three-dimensional finite element formulation of Galerkin weighted residual method. Numerical simulation is performed for a row of 25° forward-diffused shaped hole injected at 35° to an advanced gas turbine (AGT) symmetric blade. For parametric investigation, three different ramp heights, h = 0.125D, 0.2D and 0.41D, where D is the diameter of the coolant hole at inlet, along with the blowing ratios, M = 0.3, 0.5, 0.9 and 1.3 are considered, while the turbulence intensity is kept constant at 0.5%. For different combinations of blowing ratio and ramp heigh...

Numerical study of turbulent round free jet

In the present study, flow characteristics of turbulent round free jet has been studied numerically. Simulation has been done for a 29 mm diameter jet for four different jet exit velocities ranging from 19.9 m/s to 90 m/s that covers a wide range of Reynolds No. (3.94×104∼1.79×105) based on jet diameter. The main objective of this study is to evaluate the effect of jet exit velocity on mean flow properties of jet flow field (such as centerline velocity profile, jet half-width, mean velocity decay rate, jet spreading rate and self-preservation for velocity) along with turbulence kinetic energy profile and mean turbulent dissipation rate profile in the incompressible speed range (M < 0.3). Finally a two-point similarity analysis has been done to examine the well-known power-law variation and mean turbulent energy dissipation (e¯) along the axis of turbulent round jet in the context of scale-by-scale energy budget. Turbulent flow field within the computational domain has been obtained by solving Reynolds Ave...

On the prediction of explosive boiling characteristics in superheated liquid during non-equilibrium heating: Continuum modeling vs. atomistic-continuum modeling

With the recent advancement in the field of micro electro mechanical systems (MEMS), where actuation is used, researchers are in pursuit of using the energy released by explosive boiling phenomenon. Though many experimental works have been conducted on this phenomenon in the past, there have been very few theoretical works based on equilibrium nucleation theory that are unable to predict the time at which explosive boiling will occur when liquid is subjected to non-uniform transient heating. Recently two theoretical models based on two different approaches have been reported for transient non-equilibrium heating of liquid. One of the models is based on continuum assumption in which macroscopic properties of liquid together with heating condition govern the explosive boiling characteristics. The other model considers long range inter molecular forces between liquid and solid heater surface in addition to the continuum assumptions and therefore might be considered as atomistic-continuum model. In the presen...