U. H. Kurzweg

Heat transfer by high‐frequency oscillations: A new hydrodynamic technique for achieving large effective thermal conductivities

Heat transfer between two fluid reservoirs maintained at different temperatures and connected to each other via a capillary bundle is examined when the fluid within the capillaries is oscillated axially. Very large effective axial heat conduction rates, exceeding those possible with heat pipes by several orders of magnitude, are found to be achievable. A laminar hydrodynamic theory is developed to describe the phenomenon and to show that the enhanced heat conduction is one involving radial heat transfer across very thin Stokes’s boundary layers existing in these flows. Experimental measurements using water as the working fluid show effective thermal diffusivities up to 17 900 times those existing in the absence of oscillations. Since the process involves no net convective mass transport, it offers considerable promise as a means for the rapid removal of heat from radioactive fluids.

Enhanced heat conduction in oscillating viscous flows within parallel-plate channels

The hydrodynamics of enhanced longitudinal heat transfer through a sinusoidally oscillating viscous fluid in an array of parallel-plate channels with conducting sidewalls is examined analytically. Results show that for fixed frequency the corresponding effective thermal diffusivity reaches a maximum when the product of the Prandtl number and the square of the Womersley number is approximately equal to α 2 Pr = π Under such tuned conditions the axial heat transfer achievable is considerable and can exceed that possible with heat pipes by several orders of magnitude. The heat flux between different temperature reservoirs connecting the parallel-plate-channel configuration is shown, under tuned conditions, to be proportional to the first power of both the axial temperature gradient and the flow oscillation frequency and to the square of the tidal displacements. A large value for the fluid density and specific heat is also found to be beneficial when large heat-transfer rates are desired. The process discussed involves no net convection and hence achieves large heat-transfer rates (in excess of 10 6 W/cm 2 ) without a corresponding net convective mass transfer. A discussion of the physical origin for this new heat-transfer process is given and suggestions for applications are presented.

Temporal and spatial distribution of heat flux in oscillating flow subjected to an axial temperature gradient

Abstract The temporal and spatial distribution of heat flux within counter-oscillating slugs of fluid, along which is maintained a constant axial temperature gradient, is examined. It is found that the resultant axial heat flux pulsates at twice the base oscillation frequency and that the time-averaged axial heat flow under tuned conditions is orders of magnitude larger than that present in the absence of oscillations. Such thermal pumping is produced by the time-dependent interaction of a transverse conduction flux, produced by large transverse temperature gradients, with the periodic axial fluid motion.

Determination of the longitudinal dispersion coefficient in flows subjected to high‐frequency oscillations

Experimental measurements of the longitudinal dispersion coefficient in oscillatory pipe flow of binary gas mixtures at high oscillation frequencies are presented. The magnitude of the dispersion coefficient is shown to be directly proportional to the product of the square of the oscillation amplitude and the first power of frequency.

Onset of turbulence in oscillating flow at low Womersley number

Measurements on the onset of turbulence in oscillating flow of water in small diameter tubes are presented. Observations, based upon the streaming birefringence method, show that the heretofore observed radius independent onset criterion for turbulence fails to hold as the tube radius and oscillation frequency become small. In particular, it is found that with decreasing values of Womersley number, oscillating flows become increasingly stable.

Enhanced dispersion in oscillatory flows

Measurements of the dispersion coefficient in oscillating pipe flows for a wide range of Womersley numbers, tidal displacements, and tube radii are presented. The results are shown to be in good agreement with those of a recent laminar dispersion theory for oscillatory flows.

Transport of gases in high-frequency ventilation

We found that the transport of gases in oscillating gas columns was proportional to oscillation frequency and the square of oscillation amplitude. When these results were applied to high-frequency oscillation and high-frequency jet ventilation in dogs, alveolar ventilation was proportional to frequency and to the square of tidal volume, and inversely proportional to anatomic dead space.

Numerical simulation of time-dependent heat transfer in oscillating pipe flow

The problem of Enhanced Axial Heat Transfer (EAHT) in oscillatory pipe flow has been numerically studied. Time-dependent velocity and temperature profiles and Lagrangian and tidal displacements at various Wormersley numbers are obtained. The enhanced axial heat flux for different tidal displacements is calculated. The wall thickness effect in EAHT is also studied, and the optimum wall thickness is shown to be about 20% of the pipe radius in the test case studied. In addition, the tuning effect in axial heat flow is examined, the results indicating good agreement with analytic predictions. The numerical studies show that EAHT can be a very effective tool for those problems in which a large amount of heat must be transported without an accompanying convective mass exchange.

Diffusional separation of gases by sinusoidal oscillations

It is shown that gas mixtures can be separated at relatively high differential flow rates by an enhanced diffusion technique involving the oscillation of gases within a capillary bundle in the presence of axial concentration gradients. Experimental data for the diffusional separation of both He–CO2 and CO2–SF6 mixtures into an O2 carrier are presented.

Stability of swirling flows with radius-dependent density

The inviscid instability of heterogeneous swirling flows with radius-dependent density is investigated and secular relations for the instability growth rates for several different flow configurations are obtained from explicit solutions of the governing equations. It is found, in agreement with a sufficiency condition for the stability of such flows obtained earlier, that they are stable to both axisymmetric and non-axisymmetric infinitesimal modes whenever the density is a monotonic increasing function of radius and at the same time the radial variations in both the angular and axial velocity components remain small. The instability mechanisms present in these flows are both of centrifugal and of shear origin, the classical Rayleigh–Synge criterion being a condition for centrifugal stability. It is shown, via several counter examples, that the Rayleigh–Synge criterion for the stability of swirling flows is generally neither a necessary nor a sufficient condition when non-axisymmetric disturbances are considered or large shears exist in the flow. Very stable flows occur when the angular and axial velocity components have no radial variation and simultaneously the density increases with radius as is the case in a typical centrifuge.