Secondary flows and extra heat transfer enhancement of ribbed surfaces with jet impingement

Previous experiments recognize that substantial heat transfer augmentation is achieved by adding ribbed turbulators after jet impingement with cross flow present. To address fundamental working mechanisms, conjugate CFD simulations are employed for ribs, jet impingement, and their combinations. Flow characteristics and drawbacks for the individual and combined enhancement techniques are highlighted. New analysis on the coupled design arrangement reveals that the counter-rotating vortices generated by the jet flow can energize inter-rib recirculating vortices and promote span-wise convection. With an optimal design combination arrangement, extra heat transfer benefit is achieved beyond that associated with simple superposition of rib and jet impingement techniques

Permeability correction factor for fractures with permeable walls

Enhanced Geothermal Systems (EGS) are based on the premise that heat can be extracted from hot dry rocks located at significant depths by circulating fluid through fracture networks in the system. Heated fluid is recovered through production wells and the energy is extracted in a heat exchange chamber. There is much published research on flow through fractures, and many models have been developed to describe an effective permeability of a fracture or a fracture network. In these cases however, the walls of the fracture were modelled as being impermeable. In this paper, we have extended our previous work on fractures with permeable walls, and we introduce a correction factor to the equation that governs fracture permeability. The solution shows that the effective fracture permeability for fractures with permeable walls depends not only on the height of the channel, but also on the wall permeability and the wall Reynolds number of the fluid. We show that our solution reduces to the established solution when the fracture walls become impermeable. We also extend the discussion to cover the effective permeability of a system of fractures with permeable walls.R. Mohais, C. Xu, P. A. Dowd, and M. Han

Subminiature hot-wire sensor construction

The detailed procedure for constructing subminiature hot-wire sensors is given. The wire used is 0.625 micrometers diameter, 90 per cent platinum/10 per cent rhodium, plated with 30-40 micrometers diameter of silver. For construction, the plated wire is first bent into the appropriate shape, and then soldered onto stainless steel prongs. Portions of the silver are then etched, removing it entirely from a 200 mm-400 mm length of platinum/rhodium wire. Small portions of copper plating are then added to give the disired platinum/rhodium wire sensing length, and to insure that good mechanical and electrical connections exist to the sensor. With this procedure, subminiature hot wire sensors have been constructed which operatewith minimal drift, and are mechanically robust for long periods of time. The sensors have been demonstrated to be a useful research tool for turbulence research because they provide a more accurate means to measure energy levels of small-scale turbulent motions than is possible with other measurement devicessupported by the Science and Engineering Research Council of Great Britain and by the Office of Naval Research (Grant N0001 4-84-G-01 1 7) , and was carried out in the Department of Aeronautics of the Imperial College of Science and Technology, University of Londonhttp://archive.org/details/subminiaturehotw00lig

Spatial resolution and measurement of turbulence in the viscous sublayer using subminiature hot-wire probes

Measurements in the viscous sublayer of a flat-plate turbulent boundary layer in air, using single hot-wire sensors with lengths from 1-60 viscous length scales show that, at a given distance from the surface, the turbulence intensity, flatness factor, and skewness factor of the longitudinal velocity fluctuation are nearly independent of wire length when the latter is less than 20-25 times the viscous length scale (i.e. 20-25 "wall units"), and decrease significantly and abruptly for larger wire lengths. This conclusion is consistent with other workers' probability density functions of streak spacing: the lateral spacing of "streaks" in the viscous sublayer is 80-100wall units on average with minimum spacing of 20-25 wall units, which implies that signals would be strongly attenuated by wires whose length exceeds 20-25 wall units. To achieve wire lengths of less than 20-25 wall units, subminiature hot wire probes like those described by Ligrani and Bradshaw (1987), having lengths as small as 150 ~tm, are necessary for sublayer measurements in typical laboratory wind tunnels. As well as the measurements mentioned above, dissipation spectra are presented, to show the effect of spanwise averaging on the high-frequency motions, which is necessarily more severe than the effect on overall intensities

The thermal behavior of film cooled turbulent boundary layers as affected by longitudinal vortices.

http://archive.org/details/thermalbehavioro00ort