Amitesh Kumar

Development of a Theoretical Process Map for Laser Cladding Using a Three-Dimensional Conduction Heat Transfer Model

In this article, a three-dimensional conduction heat transfer model is developed to predict the clad geometry (e.g., height, width, and dilution) and microstructure (scale and morphology) of the solidified layer for a laser cladding process. The effect of controllable input process parameters like absorbed laser power, powder deposition rate, and processing speed on the clad characteristics is critically assessed with the help of dimensionless parameters. A process map is developed which enables operators to pick up the proper process parameters for a feasible laser cladding process with desirable characteristics. The present conduction model is solved using the finite-volume method in a multiblock, nonorthogonal grid system. The effect of melt pool convection is taken care of by introducing an enhanced thermal conductivity factor for the molten pool.

A novel approach to improve the efficacy of tumour ablation during cryosurgery

Freezing tumours and ablating it using cryosurgery is becoming a popular surgical procedure for treatment of carcinomas. In order to improve the efficiency of the cryosurgical procedure different approaches have been implemented till now, e.g., injecting high thermal conductivity fluid inside the tumour, low latent heat fluids inside the tumour prior to cryosurgery etc. These techniques improve the cryosurgical process to some extent but lack in minimising the damage to the surrounding healthy tissues. In this study, a novel concept is proposed which advocates the use of solutions with specific thermophysical properties around the interface of tumour. Numerical modelling has been done to determine the location of the ice fronts in the presence of this solution around the boundary of the tumour. It is noticed that in the presence of solution layer, owing to its distinct thermophysical properties like low thermal conductivity, not only the cellular destruction is enhanced but also the damage to the surrounding healthy tissue is minimised. Further, results indicate that this strategy leads to a faster ablation rate reducing the surgical time immensely. Also, an optimal offset, the minimum distance between the tip of cryoprobe and the boundary of the tumour, is identified for a given tumour radius with a given active length which gives maximum tumour necrosis in less time. This optimal offset which has been identified for each case will help the surgeons in proper planning of cryosurgery and improving the effectiveness of this technique greatly, making it a better treatment modality than its counterparts in many ways. It is also observed that for a 2 mm increase in activelength of the cryoprobe, the decrease in optimal offset is approximately 1 mm, i.e. optimal offset decreases linearly with an increase in the activelength for a given radius of the tumour. Also, for tumour with different radii, ranging between 10 mm to 15 mm, with same active length, the time taken for complete ablation by the larger tumour is nearly 2.7 times the time taken by the smaller one for every 2.5 mm increase in the tumour radius.

Mean Flow and Thermal Characteristics of a Turbulent Dual Jet Consisting of a Plane Wall Jet and a Parallel Offset Jet

The standard high-Reynolds number two-equation k − ϵ model is used to study the flow and thermal characteristics of a dual jet consisting of a plain wall turbulent jet and a parallel turbulent offset jet (hereafter, dual jet). The flow and thermal characteristics are presented in the form of streamlines, mean velocity vector, turbulent kinetic energy, dissipation of turbulent energy, Reynolds stresses, and isothermal contour plots. The variation in local heat flux and local Nusselt number on the bottom wall is also presented. The finite-volume-method-based SIMPLE algorithm is utilized for understanding the complex nature of flow arising due to a dual jet. The convective flux is discretized using the power-law upwind scheme, while the diffusive term is discretized using the central difference scheme. To study the effect of offset ratio, which is defined as the ratio of height of the jet from the horizontal wall to the width of the jet (nozzle), it is varied between 3 and 15 at an interval of 2. It is noted that the presence of a wall jet in addition to the parallel offset jet has a significant effect on flow and thermal characteristics.

Effect of cryoprotectant on optimal cooling rate during cryopreservation

The effect of initial Me2SO concentration (cgi) inside the cell lines on the optimal cooling rate is studied using a well established water transport model. A correlation formula is proposed for the determination of optimal cooling rate of freezing biological systems which depends on the cell activation energy, reference membrane permeability, initial Me2SO concentration, and the cell geometrical parameters. Here, the optimal cooling rate is defined as the highest cooling rate for which amount of trapped water inside the cell is equal to 5% of the initial cell water content at an end temperature of -40 °C. It is found that the optimal cooling rate varies linearly with the reference membrane permeability and the ratio of surface area for water transport to the initial volume of intracellular water. The developed correlation is valid for cell activation energy between 20 and 80 kcal/mole and initial Me2SO concentration between 0.1 and 1.3M. It has been observed that the optimal cooling rate does not follow a single trend for the studied initial concentration of Me2SO. However, three regions are identified within which, the variation is almost similar; the three regions are: 0.1 M ≤ cgi ≤ 0.7, 0.7 M ≤ cgi ≤ 0.9, and 0.9 M ≤ cgi ≤ 1.3M. It has been shown that the predicted optimal cooling rate is in a very good agreement with the published experimental/numerical prediction.

Melting of a Solid Sphere Placed in an Infinite Medium—Effect of Forced Convection

Heat transfer from a melting sphere due to forced convection is studied. The two-dimensional axisymmetric Navier Stokes and energy equations are solved using the finite-volume method to predict the time required for a sphere to melt in a melt pool of the same material. The heat transfer characteristics are represented by the correlation of the Nusselt number with the Prandtl number, the Reynolds number, and the Stefan number. The rate of melting of the sphere with time under different conditions is also presented.

Mechanism of morphine addiction by inhibiting the soluble Guanylate Cyclase-Nitric Oxide (sGC-NO) pathway.

Ample evidence has shown that morphine influences learning and memory and thereby causing addiction. Various studies have shown that it decreases the inhibitory GABAergic synaptic transmission (LTPGABA) via the soluble guanylate cyclase (sGC) and nitric oxide (NO) pathway. But still it is unclear on how does morphine inhibit the sGC-NO pathway. In this study, we show the mechanism of LTPGABA inhibition by morphine with the help of a mathematical model. A two step model of sGC activation is used, where morphine inhibits NO during the first step and consequently blocks sGC activation. Here, morphine binding on μ-opioid receptors blocks the binding of retrogradely travelling NO to sGC and hence its activation. As a result, LTPGABA is not produced which increases the chances of addiction manifold. Alongwith the mechanism, the dependence of morphine inhibition on major parameters such as morphine dissociation, morphine concentration, NO removal & rate of inhibition and its effect on addiction is also shown.

Melting of Steel Spherical Particle in Its Own Liquid: Application to Cladding

Blown-powder laser cladding finds its application in manufacturing industries to improve the surface properties of metallic mechanical parts. In the blown-powder laser-cladding process the powder particles go into the superheated melt pool formed by melting ofthe powder and become the integral part of the substrate coating upon solidification as the laser beam moves away. In the present study, two-dimensional axisymmetric Navier-Stokes and energy equations are solved using the finite volume method to predict the time required for a steel sphere to melt in a melt pool of the same material. The effect of forced convection, characterized by a Reynolds number, and superheat of the melt pool, characterized by a Stefan number, have been studied in detail for a Prandtl number of 0.13. The effect of buoyancy is neglected for the present investigation. It is found that the effect of forced convection on melting time reduction is more pronounced for the low Stefan number case. The rate of melting of the sphere with time under different conditions is also presented. Finally, the heat transfer characteristic is presented by the correlation of a Nusselt number with a Reynolds number and a Stefan number for a Prandtl number of 0.13. The decrease in size of the particle and its change in shape have been presented along with the evolving velocity and temperature field around the particle.