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As an augmentative means of forced convective heat transfer, relationships between the heat transfer and the flow behavior were experimentally investigated when the longitudinal vortex was artificially generated by single vortex generator in a pipe. The formation of vortices and then flow spiraling downstream were visualized by means of fluorescein and smoke as tracer of water and air flow, respectively. The velocity components were also measured using an inclined hot wire probe in three flow directions. The vortices rings being accompanied with longitudinal vortex as a kind of shear vortex gradually closed to the wall surface. It was found that the local heat transfer coefficients was mainly increased depending on the circumferential velocity component. Furthermore, a law of non-analogy between heat and flow was discussed about some locations taking accounts of both heat transfer coefficients and skin friction factors.

The temperature and the pressure distributions on a plate in underexpanded impinging jets are experimentally and numerically studied in this paper. The experiments were performed by measuring the adiabatic wall temperature using infrared imaging camera and the wall static pressures on the impinging plate and by visualizing density fields using schlieren method. Numerical calculations were conducted by solving two dimensional axisymmetric compressible Navier-Stokes equations and three dimensional compressible Euler equations. The comparison between the experiments and the calculations shows that the decrease in the stagnation temperature is caused by a recirculating flow carrying the cold air from the jet boundary to the stagnation point. The temperature and pressure ; contours on the impinging plate illustrate azimuthal structures for the circular nozzle, which are also confirmed by the three dimensional calculations., 論文

The temperature and the pressure distributions on a plate in underexpanded impinging jets are experimentally and numerically studied in this paper. The experiments were performed by measuring the adiabatic wall temperature using infrared imaging camera and the wall static pressures on the impinging plate and by visualizing density fields using schlieren method. Numerical calculations were conducted by solving two dimensional axisymmetric compressible Navier-Stokes equations and three dimensional compressible Euler equations. The comparison between the experiments and the calculations shows that the decrease in the stagnation temperature is caused by a recirculating flow carrying the cold air from the jet boundary to the stagnation point. The temperature and pressure ; contours on the impinging plate illustrate azimuthal structures for the circular nozzle, which are also confirmed by the three dimensional calculations.

A quantitative estimate of heat transfer enhancement is discussed here based on the measuements of local coefficients of heat transfer and friction for air flow over a range of Reynolds number from Re=9400 to 28200. Pairs of vortex generators consisting of two rectangular blades, increasing linearly in separation downstream, are aligned laterally to the flow with a spacing W. Characteristics of spanwise local heat transfer and pressure coefficients are found to vary differently over two regions of W. For small W, the phase of their distribution is shifted by 180° from the patterns observed just downstream of the blades. For large W, no phase change is observed but changes of the pattern between paired blades are distinctive. Overall heat transfer coefficients are more than double those for a smooth duct without generators. As a result, the performance ratio is greater than unity for all values of W., 論文

The longitudinal vortices were artificially generated by a single winglet vortex generator in a pipe. The longitudinal vortex enhances the heat transfer compared to other vortices produced by protrusions, because this longitudinal vortex does not decay until relatively further downstream. The purpose of this study is to know the informations about the flow motion of longitudinal vortex and the influence of vortices on heat transfer enhancement. The flow pattern was visualized by means of both fluorescein and rhodamine B, as a tracer of the water flow in a water table. The main vortex was spirally moved along the circumference and the behavior of other vortices was observed. Streamwise and circumferential heat transfer coefficients on the wall, wall static pressures, and velocity distribution in the overall cross section were also measured for air flow in the range of Reynolds number from 18 800 to 62 400. The distributions of streamwise heat transfer coefficients had a periodic pattern, and the peaks in the distribution moved circumferentially due to the spiral motion of the main vortex. Relation of the iso-velocity distribution and the wall static pressure to the heat transfer characteristics was shown. Moreover, the thermal performance was calculated under the condition of fixed pumping power, and it was found that the performance ratio is almost larger than unity., 論文

Heat transfer characteristics and flow patterns were measured over the plate for various separation distances between the nozzle exit and target plate when the air issuing from a sharpedged cross-shaped nozzle impinges on the plate. The local heat transfer coefficients in the radial direction for different circumferential positions were calculated using the wall temperatures measured by means of thermocouples, and the flow patterns were observed using a oil-titanium IV oxide method. The isotherms of the infrared images were also measured using an infrared radiometer with a two-dimensional array of InSb sensors. The geometric axes were switched due to the self-induced velocity of a vortex filament ; the convex corners became flat and the concave corners generated outward ejection. The distribution of the iso-heat-transfer-coefficient contours corresponds well to the flow pattern and the isotherm contours. These contours extended diagonally and demonstrated the Andrew's cross pattern for the short separations, subsequently becoming the octagonal patten, and then becoming circular at large separations. The correspondence of the heat transfer characteristic to the flow behaviors, and the heat transfer mechanism were also described., 論文

The characteristics of heat transfer and pressure distributions were measured by varying the clearances against the duct wall and the separations of the two thin plates for various arrangements of the thin plates which were oscillated by a flow in a parallel plate duct, with a view to enhancing the heat transfer along the duct. The maximum and mean Nusselt numbers have a Reynolds number dependence of Re^, and were 2.3 and l.6 times those in the fully developed turbulent flow, respectively, for air with Reynolds number ranging from 9000 to 37500. A relatively new technique, i.e., full field infrared imaging, was used to obtain the time and spatial temperature profiles on the wall surface. Contours of the isotherm of the infrared images correspond well to the heat transfer characteristics and flow behavior., 論文

The heat transfer coefficients and the pressure distributions were measured downstream of the backward facing step in which the heat transfer was actively or passively enhanced by following procedure. (1)The jet was ejected perpendicularly to the main flow from the portion on the opposite wall of the step. (2) The air flow was sucked from the recirculating zone. (3) A slat was inserted with an inclination to the main flow. These cause the main flow to curve sharply, and then the recirculating zone to shorten, so it is easy to control the lengths of the reattachment point, and consequently, the heat transfer. The thermal performance, η defined as the ratio of the heat transfer coefficient at the recirculating region with these modifications to that of a normal region, was calculated for equivalent pumping power, and it was found that η was larger than unity for the enhanced systems, except in the case of the discharge jet. It is effective to apply the enhancement of the heat transfer by active or passive methods to the suddenly expanding tunnel., 論文

This paper shows how the heat-transfer coefficients along heated tubes are affected by the clearance against the tube wall, the inlet angle and the widths of various types of blade wheels which are propelled by an internal flow in a circular tube and inserted at the inlet in order to augment heat transfer along the tube. The maximum and averaged heat-transfer coefficients in the region from blade position to far downstream were 3.0 and 2.2 times those in smooth duct respectively, for air ranging in Reynolds number from 15000 to 60000. Moreover, the performance evaluation was carrid out under the condition of fixed pumping power, and it was found that the performace ratio depends more on the inlet angle and the width of the blde than clearence, and the ratio is larger than one., 論文