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J. M. F. VICKERS' University of Toronto, Toronto 5, Ontario, Canada
A n Unusual Flow Pattern M a y Explain the Behavior of
Heat Transfer Coefficients between Fluid Jets and Normal Surfaces I N MANY ENGINEERING applications, laminar jets of air or other fluids striking normal surfaces are used to cool or heat either the surface or fluid. However, measurement of point heat transfer coefficients has been neglected until recently, and the fluid-flow effects involved are probably not well understood ( 2 2 ) .
Experimental The point where the heat transfer coefficient was measured was defined by its distance from the stagnation point of the jet which is that point where the velocity of an impinging stream line is reduced to zero, and its kinetic energy is entirely converted into pressure and thermal energy. This stream line is along the polar axis of the jet when a threedimensional, axially symmetric jet strikes a plane surface normal to its direction of flow. The stagnation point is, therefore, the point a t which the polar axis of the jet intersects with the target plane. Apparatus. The target plane was of extra rigid Aero-Jablex cut into cylinders 11/* inches long and either 0.79 or 1.58 inches in diameter ( A , Figure 1). Longitudinal holes were drilled at various radii from the center of the top surface (Figure 2). Copper-constantan thermocouples (30 B and S gage) were inserted into the holes, with the tips of the junctions just above the surface of the material. The surface was then covered with three coats of insulating varnish, the complete assembly glued rigidly to the brass boss ( B ) ,and mounted in a surface grinder. The complete target surface was then ground smooth, giving a nonporous surface with the junctions of the thermocouples forming a part of the surface itself. Each thermocouple, after grinding, was an effective point sink to the radiant energy used to heat the target-its cross section was a center core of copper, 0.014 inch in diameter, surrounded by a ring of constantan, 0.035 inch in outside diameter. The prepared surface and inner surface of the electrically heated furnace (D)were covered with lamp 1 Present address, University of Nebraska, Lincoln 8, Neb.
black in alcohol, so that surface emissivity of the emitting and absorbing surfaces was accurately known (70), while the periphery of the target cylinder was covered with aluminum foil to reduce the radiant heating of the remainder of the material. The target and boss, mounted on the pedestal (C), could be rotated through 360' and moved vertically through about 1.25 inches. The furnace (D), made from a standard 5-inch steel tube, was supported by insulating blocks ( H ) on the stand ( E ) , and completely surrounded the target. The brass rod in which the orifice ( F ) was drilled was insulated from the furnace top by an annulus of wood, and the lower surfaces of the brass tube and ofthe wood annulus were covered with aluminum foil. The form factor from the furnace to the target was calculated using a form factor graph (9). The furnace was an effective isothermal surface for heating the target, because thermocouples, attached to the inner wall and top of the furnace and covered with lamp black, indicated surface temperature variations of about 0.5 %. Procedure. The air supply (Figure 3) was taken from a reciprocating air compressor through an oil filter ( A ) to a calibrated gasometer of 2 cubic feet capacity ( B ) , and then via a capillary tube (C) and a drying tower ( D ) to the orifice tube in the apparatus (E). An inclined manometer ( F ) determined pressure drop across the capillary tube (C) and this drop was used to ensure that the flow rate was constant. The actual flow rates in the apparatus were determined from the time taken to consume a given quantity of air from the gasometer, measured with a stop watch. The pressure in the gasometer was read by means of the manometer (G). Output of the thermocouples was measured with a Negretti and Zambra quick-reading potentiometer, having a range 0 to 41 mv. in intervals of 0.02 rnv. The cold junctions of the individually calibrated thermocouples were placed in a mixture of ice and water, and the change-over for connecting the thermocouples to the potentiometer was made by a series of copper plugs and
sockets to ensure that no further cold junctions were made. The target was considered to lie in a region of stratified air in the upper portion of the furnace, and natural convection effects were small. The stratification was well defined when the interior of the furnace was explored with a shielded thermocouple when no air was flowing in the jet. Under these circumstances, equilibrium with no air flow could be obtained when the heat reception from the furnace was exactly equal to the heat loss from the target. These inherent losses were checked first by placing a thermocouple in the target material, 0.25 inch from the surface of the target and 0.25 inch from the side of the cylinder. This gave a reading which was sensibly constant over a
Figure 1. The target plane was of extra rigid Aero-Jablex, an aerated plastic with low thermal conductivity and high electrical resistance
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