Two unsteady heat transfer experiments. Part 1: In grid-generated isotropic turbulence. Part 2: In laminar oscillatory flow in straight and conical tubes

A screen of closely spaced, parallel, thin wires was placed downstream of a grid generating nearly isotropic turbulence. The screen was normal to the flow and was heated in one of two modes: (1) periodically in time, to generate a train of transversely uniform streamwise thermal ramps, each with a uniform streamwise gradient; and (2) steadily, with transverse nonuniformity, to generate a uniform transverse thermal ramp. The simple temperature and temperature gradient fluctuation statistical properties in both cases were comparable to those encountered with a steadily heated grid producing a uniform transverse thermal ramp. In both modes of heating the temperature fluctuations decreased initially behind the screen and then increased monotonically. The skewness of the temperature fluctuation derivative in the direction of the main gradient was found to be nonzero despite the absence of mean shear. Experiments on the flow field and the dispersion of heat in laminar oscillatory flow in straight and conical around tubes obtained the fluid point displacements by hydrogen bubble visualization in water and the velocity field by hot-wire anemometry in air. The heat lost through the heating frame and the insulated walls of the tube is estimated from wall temperatures extrapolated from radial profiles in the flow. The terms in the energy equation roughly balance. A plug oscillatory flow model theoretical model generates temperature fluctuations which resemble those in the core of the experimental flow.