Unsteady mixed convection flow at the stagnation point

The unsteady mixed convection flow of an electrically conducting fluid at the stagnation point of a two-dimensional body and an axisymmetric body in the presence of an applied magnetic field has been studied. The effect of induced magnetic field has been included in the analysis. Both prescribed wall temperature and prescribed heat flux conditions have been considered. It is found that if the free stream velocity, applied magnetic field and square root of the wall temperature vary inversely as a linear function of time, i.e. as (1 − λt′)⁻¹, the governing boundary layer equations admit a locally self-similar solution. If surface heat flux is prescribed, it should vary as (1 − λt^*)^−5/2 for the existence of a local self-similar solution. The resulting ordinary differential equations have been solved using a finite element method as well as a shooting method with Newton's corrections for missing initial conditions. The skin friction and heat transfer coefficients and x-component of the induced magnetic field on the surface increase with the applied magnetic field or buoyancy force. Also they are found to change more for decelerating free stream velocity than for accelerating free stream velocity. Furthermore, they change little with the reciprocal of the magnetic Prandtl number. The buoyancy parameter causes overshoot in the velocity profile. For a given Prandtl number, beyond a certain critical value of the dissipation parameter, the hot wall ceases to be cooled due to the “heat cushion” provided by frictional heat.     

Copolymer characterization by surface tension

The use of surface tension measurements is proposed as a simple method for the determination of copolymerization ratios. The procedure depends on the parachor of the copolymer in the liquid state (∏), which is defined by ∏ = ∏_s + ∏₀(w_p/w_s)(M_s/M₀) for solutions and ∏ = [DP]∏₀ for liquid or molten polymers, where ∏_s is the parachor of the solvent of molecular weight M_s; w_p and w_s are the weights of the polymer and solvent in solution; while ∏₀ and M₀ are the parachor and molecular weight of the repeating unit of the copolymer, respectively. The validity of this relationship is demonstrated by analysis of the surface tension properties of liquid silicone polymers, polystyrene–Decalin solutions, and molten polymers as well as by the calculation of the composition of some characterized tetrahydrofuran–propylene oxide copolymers. The application of surface tension measurement is also suggested as a means of estimating the degree of chain branching in a polymer.     

Gas lasers

The important gas lasers which find wide applications in material processing are the CO₂ laser, the argon-ion laser and the excimer lasers. This paper briefly describes the basic concepts and the technology of these lasers.     

Preionizer-induced pulsed-mode operation of a transverse-flowtransversely excited CW CO2 laser

A pulse-preionized, dc-excited, transverse-flow, CW CO₂ laser has been operated in pulsed mode without using a discharge power-switching scheme. The pulsation in laser power is produced by a pulsed current that flows from the dc power supply due to the collapse in the discharge impedance whenever a preionizer pulse appears. By properly choosing the preionizer-pulse repetition frequency, dc power-supply filter-capacitor value, and gas composition, the laser pulsation has been enhanced. This laser has been operated at 1-kW average laser power level in pulsed mode at 1.3-kHz pulse repetition frequency with a peak-to-average power ratio of 3.35 and laser-power modulation depth of almost 100%     

Effect of large blowing rates on the compressible boundary layer flow at the stagnation point of a rotating sphere

Summary The effect of a large surface blowing (injection) rate on the steady laminar compressible boundary-layer flow at the forward stagnation point of a rotating sphere has been studied. The resulting coupled nonlinear ordinary differential equations have been solved using two methods, namely, the method of matched asymptotic expansion and the implicit finite difference scheme in combination with the quasilinearization technique. It is found that the boundary layer thickness increases considerably with the blowing rate. The location of the dividing streamline moves away from the surface with increasing blowing rates, but moves towards the surface when the total enthalpy at the wall or the rotation parameter increases. For large blowing rates and small rotation parameter the surface heat transfer and the surface shear stress in the tangential direction tend to zero, but the longitudinal shear stress remains finite but small. Also, for this case, the longitudinal shear stress at the wall is approximately found to be directly proportional to the sum of the total enthaply at the wall and to the square of the rotation parameter and inversely proportional to the blowing rates. The rotation parameter induces overshoot in the longitudinal velocity, and the magnitude of the velocity overshoot increases significantly with rotation and blowing parameters. However, there is no overshoot in the longitudinal velocity in the absence of rotation whatever may be the values of the blowing parameter.