Muhammad Yousaf | Purdue University (original) (raw)

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Papers by Muhammad Yousaf

Gas-liquid two-phase flows are extensively observed in many industries due to their tremendous ap... more Gas-liquid two-phase flows are extensively observed in many industries due to their tremendous applications. The examples include but are not limited to the chemical, petroleum, and nuclear industries [1]. Transport of gas-liquid in large size pipes significantly affects the performance of the pumps. There has been an extensive research on two-phase flows in the small diameter pipes both in up and downward flows. But the research related to large diameter pipes are limited mainly for downward flows. On the other hand, the drift-flux model (DFM) is extensively utilized to analyze and predict the two-phase flows. Due to the dependence of the two-phase flows on the geometry, correlations for large and small diameter pipes significantly differ. In the present research, drift flux model analysis was carried out for the downward two-phase flows in 4in and 8in pipes. Results showed that the size of
pipes affects the two-phase flows. Moreover, experimental
data showed that Goda’s DFM correlations are in good
agreement with the 4in pipe while, deviate from the 8in pipe.

Downward two-phase flow in large diameter pipes appears in numerous industrial applications and n... more Downward two-phase flow in large diameter pipes appears in numerous industrial applications and nuclear reactor accidents. In this study, adiabatic air–water two-phase flow experiments in a 203.2 mm diameter pipe have been conducted to investigate flow regimes and their transitions in downward and horizontal flow. Three flow regimes (cap-bubbly, churn-turbulent and annular flow) were recognized in downward flow, as well three flow regimes (stratified, plug and pseudo-slug flow) were observed in horizontal section. Evolution of void fraction and flow structure along the loop under different flow conditions has been discussed. The Probability Density Function (PDF) and Cumulative Probability Density Function (CPDF) of area-averaged void fraction signals were utilized as the indicators for self-organized neural network (SONN) method to identify horizontal and vertical downward flow regimes, respectively. The downward flow regime maps for 203.2 mm diameter pipes have been proposed and compared with that for different diameter pipes. The results show that the flow regime maps agree well with that of 101.6 mm, but don't agree well with that of smaller diameter pipes (25.4 mm and 50.8 mm). It is found that the transition between churn-turbulent and annular flow occurs at a certain superficial liquid velocity regardless of superficial gas velocity. A set of new transition criteria have been developed for downward flow regime transitions in large diameter pipes, and validated by the experimental data of 203.2 mm and 101.6 mm diameter pipes. Compared with existing models, these criteria provide more accurate predictions for downward flow regime transitions in large diameter pipes.

Downward two-phase flows in large diameter pipes are important in various industrial applications... more Downward two-phase flows in large diameter pipes are important in various industrial applications, especially for the safety analysis in nuclear reactors. To address the issue that few data of downward flow in large diameter pipes is available for model evaluation, experiments of air–water downward flow in a pipe with inner diameter of 203.2 mm have been performed. Area-averaged void fraction and pressure measurement, as well as flow visualization, have been conducted at several axial locations. The flow conditions for superficial gas velocity range from 0.05 m/s to 3.00 m/s and for superficial liquid velocity range from 0.1 m/s to 1.5 m/s, which cover cap-bubbly flow, churn-turbulent flow and annular/falling film flow. The flow structure at several axial locations and the transition from churn-turbulent flow to annular/falling film flow have been discussed. Current available drift-flux models developed for downward flow in regular pipes as well as for upward flow in large pipes are evaluated using newly collected data. For churn-turbulent flow, the data indicates a larger drift velocity than the model prediction. Corresponding drift-flux constitutive equations are suggested which can reduce the prediction error from 34.37% to 11.79%.

Simulation code for detector deadtime was developed and tested to investigate the performance of ... more Simulation code for detector deadtime was developed and tested to investigate the performance of various deadtime models.Two ideal deadtime and two hybrid models were compared by simulating a decay source experiment.Both hybrid models are always fall between the two ideal models.The two seemingly similar hybrid models produce significantly different results.High intensity radiation measurements are confounded by detector dead-time and pulse pile-up problems. A computational method was used to compare the traditional dead-time models with recently proposed hybrid dead-time models. A computational algorithm based on a decay source method was used to study the behavior of various dead-time models. Validation of the code was performed for the hybrid models by confirming that the predictions lie between the two ideal dead-time models; the paralyzing and the non-paralyzing model. It was interesting to note that two seemingly similar hybrid dead-time models produced significantly different results. Lee and Gardner's model based on two dead-times and Patil and Usman paralysis factor based model are inherently different in their logic as well as results. For Lee and Gardner's model altering the orders of dead-times produced significantly different response. These hybrid models should be studied further to investigate both the dependence and the variation of model parameters on detector design and operating conditions. It is well accepted that one dead-time does not apply to all detectors and even for the same detector applicability of the same model under all operating condition is questionable. Therefore, dead-time model should be chosen carefully for the specific detector, operating conditions and radiation to be measured to correctly represent the physical measurement phenomenon.

Natural convection in a two-dimensional square cavity in the presence of roughness on vertical wa... more Natural convection in a two-dimensional square cavity in the presence of roughness on vertical walls was
studied numerically. A single relaxation time Bhatnagar–Gross and Krook (BGK) model of Lattice
Boltzmann method (LBM) was utilized to solve coupled momentum and energy equations. Validation
of computational algorithm was performed against benchmark solutions, and a good agreement was
found. Numerical study was performed for a range of the Rayleigh number from 103 to 106 for a
Newtonian fluid of the Prandtl number 1.0. The sinusoidal roughness elements were located on a hot,
and both the hot and cold walls simultaneously with varying number of elements and the dimensionless
amplitude. Hydrodynamic and thermal behavior of fluid in the presence of roughness was analyzed in
form of isotherms, velocity streamlines, and the average heat transfer. Results based on this numerical
study showed that the sinusoidal roughness considerably affect the hydrodynamic and thermal behavior
of fluid in a square cavity. A dimensionless amplitude of sinusoidal roughness elements approximately
equal to 0.025 has no significant effects on the average heat transfer. The maximum reduction in the
average heat transfer was calculated to be 28% when the sinusoidal roughness elements were located
on both the hot and cold walls simultaneously.

The present study focused on investigation of the effects of roughness elements on heat transfer ... more The present study focused on investigation of the effects of roughness elements on heat transfer during natural convection in a rectangular cavity using numerical technique. Roughness elements were introduced on the bottom hot wall with a normalized amplitude (A*/H) of 0.1. Thermal and hydrodynamic behavior was studied using computational method based on Lattice Boltzmann method (LBM). Numerical studies were performed for a laminar natural convection in the range of Rayleigh number (Ra) from 10 3 to 10 6 for a rectangular cavity of aspect ratio (L/H) 2 with a fluid of Prandtl number (Pr) 1.0. The presence of the sinusoidal roughness elements caused a minimum to maximum decrease in the heat transfer as 7% to 17% respectively compared to smooth enclosure. The results are presented for mean Nusselt number (Nu), isotherms and streamlines.

GM counter deadtime dependence on applied voltage, operating temperature and fatigue. Standard tw... more GM counter deadtime dependence on applied voltage, operating temperature and fatigue. Standard two-source method and simple non-paralyzing model assumption. Three distinct regions of the deadtime behavior with low deadtime plateau in the middle. Exponential increase in deadtime with increasing temperature. Increase in deadtime with fatigue.

High intensity radiation measurements are confounded by detector dead-time and pulse pile-up prob... more High intensity radiation measurements are confounded by detector dead-time and pulse pile-up problems. A computational method was used to compare the traditional dead-time models with recently proposed hybrid dead-time models. A computational algorithm based on a decay source method was used to study the behavior of various dead-time models. Validation of the code was performed for the hybrid models by confirming that the predictions lie between the two ideal dead-time models; the paralyzing and the non-paralyzing model. It was interesting to note that two seemingly similar hybrid dead-time models produced significantly different results. Lee and Gardner's model based on two deadtimes and Patil and Usman paralysis factor based model are inherently different in their logic as well as results. For Lee and Gardner's model altering the orders of dead-times produced significantly different response. These hybrid models should be studied further to investigate both the dependence and the variation of model parameters on detector design and operating conditions. It is well accepted that one dead-time does not apply to all detectors and even for the same detector applicability of the same model under all operating condition is questionable. Therefore, dead-time model should be chosen carefully for the specific detector, operating conditions and radiation to be measured to correctly represent the physical measurement phenomenon.

Numerical studies were conducted using Lattice Boltzmann Method (LBM) to study the natural convec... more Numerical studies were conducted using Lattice Boltzmann Method (LBM) to study the natural convection in a square cavity in the presence of roughness. An algorithm based on a single relaxation time Bhatnagar-Gross-Krook (BGK) model of Lattice Boltzmann Method (LBM) was developed. Roughness was introduced on both the hot and cold walls in the form of sinusoidal roughness elements. The study was conducted for a Newtonian fluid of Prandtl number (Pr) 1.0. The range of Ra number was explored from 10 3 to 10 6 in a laminar region. Thermal and hydrodynamic behavior of fluid was analyzed using a differentially heated square cavity with roughness elements present on both the hot and cold wall. Neumann boundary conditions were introduced on horizontal walls with vertical walls as isothermal. The roughness elements were at the same boundary condition as corresponding walls. Computational algorithm was validated against previous benchmark studies performed with different numerical methods, and a good agreement was found to exist. Results indicate that the maximum reduction in the average heat transfer was 16.66 percent at Ra number 10 5 .