M. Ruhul Amin

Conjugate heat transfer during two-phase solidification process in a continuously moving metal using average heat capacity method

Abstract The average heat capacity method was successfully implemented for the analysis of conjugate heat transfer during the two-phase solidification process in continuous castings. The finite element technique with fixed grid was used for the current research. This method overcomes the limitation of the time increment of the otherwise so-called effective heat capacity method. The investigation included the ranges of mold cooling rate (Bi2) , post-mold cooling rate (Bi3) , superheat (Θo) , and withdrawal speed (Pe) from 0.025 to 0.075, 0.042 to 0.126, 1.2 to 2.7, and 1.0 to 3.45, respectively. The current study shows that two parameters, namely, Pe and Θo are very important in controlling the microstructure and breakout condition of the cast material.

Thermal Analysis During Continuous Casting Process Using Effective Heat Capacity Method

A numerical analysis has been carried out to investigate the two-phase solidification process in continuous castings. An effective heat capacity method is used for this purpose. The radiation heat transfer at the mold metal interface was taken into account. The results of this method match well with the results obtained by analytical methods. The investigation included the ranges of mold cooling rate Bi 2 , inlet temperature θ 0 , and Peclet number Pe from 0.1-0.5, 1.2-2.0, and 0.1-1.0, respectively. A constant Stefan number Ste of 2.5 was used. It is observed that the solidification process is delayed with the increase of the withdrawal speed and with the increase of the liquid metal inlet temperature. With increased withdrawal speed, the region with temperature gradient moves downward. Steep axial temperature gradient was observed in the cast metal in the mold region.

EFFECTS OF MULTIPLE OBSTRUCTIONS ON CONJUGATE FORCED CONVECTION HEAT TRANSFER IN TUBES

A numerical investigation has been carried out for conjugate forced convection heat transfer in tubes with multiple obstructions. The investigation included the ranges of Reynolds number Re, thermal conductivity ratio kr, and the distance between two obstructions d of 50-600, 0.3-3.0, and 2.0-8.0, respectively. A fluid Prandtl number Pr of 7.56 is used. The obstructions create recirculation flow / back flow in the tube with increase and decrease of local heat transfer rates. Reversal in the direction of heat transfer occurs at certain locations of the tube due to the presence of the obstructions. At high values of Re and d, reversal in the direction of heat transfer occurs on the outer wall after the second obstruction. The overall heat transfer rate decreases with the decrease of kr.

Aerodynamic Factors Affecting Performance of Straight-Bladed Vertical Axis Wind Turbines

Unlike the conventional aerodynamic applications, the straight-bladed vertical axis wind turbines (VAWTs) operate in a circular motion and encounter a wide range of angle of attacks, especially at low tip speed ratios. When the blade angle of attack remains constant or varies slowly with time, it encounters the static stall. However, when the angle of attack changes rapidly with time, it experiences the dynamic stall which is far more difficult to analyze and predict than the static stall. Furthermore, the blade/blade wake interaction in straight-bladed VAWTs also presents modeling problem. In this paper, all of these aforesaid aerodynamic factors are discussed. It was found that these factors need special attention for designing a self-starting straight-bladed VAWT with optimum performance. A numerical method based on Cascade model, proposed by Hirsch and Mandal [1], that gives reasonable correlation with the experimental data available has been used. The effects of dynamic stall and flow curvature on the performance of a straight-bladed VAWT have been analyzed. It is observed from the analysis that aerodynamic forces due to dynamic stall are higher than those due to static stall. As a result, for the performance prediction of straight-bladed VAWTs, especially for the local forces, there can be substantial differences between the experimental data and the calculated values unless the dynamic stall effect is added.Copyright

The effect of adiabatic wall roughness elements on natural convection heat transfer in vertical enclosures

The natural convection heat transfer in a two-dimensional vertical enclosure fitted with a periodic array of large rectangular elements on the bottom horizontal wall is investigated numerically. The numerical investigation is carried out for the ranges of Rayleigh numbers (Ra[inH]) and aspect ratios (W/H) of 1.85 × 102−1.85× 107 and 0.4–2.4, respectively. Prandtl numbers (Pr) of 0.72 and 4.52 are considered. It is found that the large roughness elements reduce the heat transfer rate across the enclosure. The reduction in heat transfer is more significant for enclosures with W/H > 1. The highest value of heat transfer reduction obtained in this research is 44%. This is obtained for an enclosure with W/H = 2.4 at RaH= 1.85 x 104 andPr = 4.52,

MIXED CONVECTION AND ENTROPY GENERATION CHARACTERISTICS INSIDE A POROUS CAVITY WITH VISCOUS DISSIPATION EFFECT

This paper examines and explains two-dimensional, steady mixed convection flow in a porous square vented cavity. The interaction between the buoyancy stemming from one or more heated elements inside a microstructure filled vented enclosure and an imposed forced flow forms the topic of this investigation. Analysis has been carried out for two different boundary conditions. Initial investigations are carried out for walls of the enclosure being isothermal. A second stage of analysis is performed keeping only the left wall isothermal and other three walls adiabatic. Natural convection takes place due to temperature difference between the isothermal wall and the fluid. Forced convection condition is imposed by providing an inlet and a square vent inside the enclosure filled with fluid saturated porous medium. The mathematical model is developed using modified Darcy flow model and energy equation. Through the adaptation of the well known finite element method, solution to this numerical problem is obtained. Governing parameters chosen are Peclet Number (Pe), Rayleigh Number (Ra), Aspect ratio (AR) and the width of the inlet as a fraction of the width (I/W) of the enclosure. For detailed analysis different value of these parameters such as five Rayleigh Numbers (1, 50, 100, 500 and 1000) and seven different Peclet Numbers (0.1, 1, 5, 10, 20, 50 and 100) are considered. Effect of inlet to cavity width ratio is examined within the range 0.1 ≤ I/W ≤ 0.5 for a particular aspect ratio. The performance of the enclosure in both cases; are determined by flow visualization and by analyzing different parameters such as Bejan Number, Nusselt Number and Entropy Generation Number. Isotherms, streamlines show substantial variation in their pattern or magnitude. Average Nusselt number and average Bejan Number increases whereas Average energy flux density decreases with increasing I/W. These fluctuations also vary for different Rayleigh or Peclet numbers. The results for both the boundary conditions are also compared to find the most effective value of I/W.Copyright

Natural convection heat transfer in enclosures fitted with a periodic array of hot roughness elements at the bottom

The natural convection heat transfer in a two-dimensional rectangular enclosure fitted with a periodic array of hot roughness elements at the bottom is investigated numerically. The numerical investigation is carried out for the ranges of Rayleigh numbers (RaH) and aspect ratios (W/H) from 102 to 105 and from 1.0 to 4.0, respectively, at a Prandtl number (Pr) of 0.707. Increase in heat transfer is obtained when the roughness element phase shift is equal to half its period. The increment in heat transfer is more significant for enclosures with higher values of roughness element amplitude. The highest value of heat transfer enhancement obtained in this research is 78%. This is obtained for an enclosure with W/H = 4.0 at RaH = 102.

Effects of multiple obstructions on natural convection heat transfer in vertical channels

Abstract Free convection flows in vertical channels with two rectangular obstructions on opposite walls was studied numerically. Four different geometries were employed to study the effect of the parameters like Rayleigh number ( Ra ), aspect ratio ( Ar ), and obstruction locations ( L 1 , L 2 ) on the average Nusselt number ( Nu ). The results included isotherms, streamlines and Nusselt numbers. Average Nusselt numbers for the obstructed channels were less than those of the smooth channel. The maximum reduction in heat transfer found in this study is approximately 31% and occurred at Ra = 5 × 10 2 . The minimum reduction in heat transfer is approximately 3% and occurred at Ra = 10 4 .

NUMERICAL INVESTIGATION OF THE EFFECTS OF TURBULENCE FROM SUBMERGED ENTRY NOZZLE DURING CONTINUOUS CASTING PROCESS

A numerical investigation was performed using Prandtl mixing-length theory along with the average heat capacity method to model a turbulent plane submerged entry nozzle continuous casting process. This method features the simplicity of a zero-equation model along with ease of maintaining the time-step increment. Results from the study match well with other published data. The current study indicates that submergence depth of the entry nozzle affects the turbulence level and hence controls the quality of the finished product. The inlet speed of the molten metal also controls the solidification front slope and the surface heat flux of the caster.

Conjugate forced convection heat transfer in tubes with obstruction

q0 along the interface, thermal conductivity of the e uid, thermal conductivity of the solid, outside wall temperature, e uid inlet temperature, average e uid velocity at the inlet, e uid density and e uid viscosity, respectively. The average dimensionless heat e ux F was determined by integrating Fx along the solid ‐ e uid interface. The computer code was tested for its accuracy by comparing the results with other benchmark solutions. A grid independency test was also conducted. Details of the code validation tests are documented in Amin. 4