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The thermal stability of a layer of water between two vertical parallel plates maintained at different temperatures, T1 and T2, is investigated for the conduction regime. Three cases are considered include where: (a) the maximum density layer is within the water region; (b) is at the boundary; and (c) is outside the water region. Cubic polynomials are used to represent the density-temperature relation in the temperature range 0–55°C. The numerical results show that the critical states of stability do not depend on Prandtl number but instead depends on the combinations of T1 and T2. Of the three cases considered, the first case is the most unstable. In all three cases, the instabilities set in as a traveling wave, moving against gravity, for most T1 and T2 combinations. Stationary waves were also found for case (c).

The present study examined the effects of pulsed direct current (dc) on the performance of water electrolysis for hydrogen generation. The laboratory effort consisted of a series of tests using the Fm01‐LC electrolytic cell, manufactured by ICI Chemicals and Polymers, and rated at 38 g of H2/normalkA·normalh . Using a 10 weight percent sulfuric acid solution, the electrolytic cell performance was examined using nonpulsed dc, to determine the cells base‐line performance and pulsed dc. The pulsed dc test runs were carried out using a square pulse for seven frequencies, ranging from 10 Hz to 40 kHz, and four duty cycles of 10, 25, 50, and 80%. The hydrogen generation rate was used instead of current in order to facilitate an accurate comparison. For the range of tested parameters, the results demonstrated that the nonpulsed dc operation required the least electrical power.

The effects of nonuniform volumetric energy sources on the thermal stability of natural convection in a vertical fluid layer are investigated using the linear theory of stability. The neutral states of stability are obtained for four values of Prandtl number, ranging from 0.71 to 100, and for different values of β in the range 0.1 ≤ β ≤ 10. The critical parameters and the energetics of the critical disturbances are determined for each case. For all values of Prandtl number and β studied here, the critical instabilities set in as traveling waves, moving in the direction of gravity. In the low range of Prandtl number, 0.71 to 5, the critical curves approach the same asymptotic value for β ≥ 10. The critical disturbances are found to be buoyancy driven for all Prandtl numbers and β considered here, except for the low range of Prandtl number, for which they become shear driven disturbances as β increases.

A computational fluid dynamics (CFD) model investigating the heat transfer convective coefficient of the upstream face, the upstream face and the tube face, and the upstream face, tube face, and leeward face of a perforated sheet was developed. This model was based on the hexagonally shaped flow pattern that exists around each of the holes in a perforated sheet of a certain pitch to diameter ratio. The CFD model used in the investigation of the convective heat transfer coefficient involved a single hole in a thin hexagonally shaped sheet with appropriate boundary conditions. Through a series of models varying the inlet velocity, hole diameter, and the plate temperature and then solving for the exit temperature the convective coefficient could be obtained. After obtaining the convective coefficient, the Nusselt number was calculated. These values were then plotted against the Reynolds number and an equation for the line was obtained of the form: Nu=C1·ReC2(1)

In this paper, experimental investigation has been performed to characterize the heat transfer behavior of CuO–water and ZnO–water nanofluids. Nanofluids containing different volume percent (vol %) of nanoparticle concentrations flowed over a flat copper plate under a constant heat load. The constant heat flux was maintained using evenly placed cartridge heaters. The heat transfer coefficients of nanofluids were measured and compared with the results obtained from identical experiments performed with de-ionized (DI) water. In order to thoroughly characterize the nanofluids, nanoparticle size was investigated to inspect for possible agglomeration. The particle size was measured by using both a transmission electron microscope (TEM) and a dynamic light scattering system (DLS). Enhancement of convective heat transfer of nanofluids was 2.5–16% depending on the nanoparticle concentrations and Reynolds number. The plausible mechanisms of the enhanced thermal performance of CuO and ZnO nanofluids will be discussed in the following paper.

Thermal characteristics of CopperII and Zinc-Oxide nanofluids have been investigated for flow over a heated flat plate. Fluid containing different volume-percents nanoparticles flow over a heated flat plate at specified laminar flow velocities. The heated plate experienced a constant heat flux from cartridge heaters spaced evenly along the length of the plate. The flow channel’s cross-sectional area was a square of dimensions 5cm × 5cm. Investigation of the heat transfer occurring in a plane along the centerline of the plate in the direction of the flow was performed. The heat transfer coefficients were calculated and plotted verses the Reynolds number and compared with the results obtained from distilled de-ionized water. Nanoparticle size from the fluids was investigated to inspect for possible agglomeration using both a transmission electron microscope (TEM) and a dynamic light scattering system (DLS).