OCR Text |
Show The significant result of this equation is that for circular jets, recirculation increase with K (kinematic momentum). Since FM temperature control has a greater ratio of momentum flux to energy input rate at conditions of reduced heat input, FM temperature control may enhance the convective coefficient. In addition to the FM control mode producing a higher momentum flux than is attained with the AM mode, the FM mode introduces a turbulence enhancement mechanism that may be significant in promoting convective heat transfer in high temperature thermal processes. In a turbulent boundary layer, energy is transported across streamlines by the turbulent eddies that develop in the flow field. It has been reported in the literature (Schlichting!-^) that the major contribution to the dissipation of energy in the boundary layer is the small higher frequency eddies located in a narrow strip inside the boundary layer. In consideration of the above, enhancing the higher frequency components of a flow field will enhance the convective heat transfer across the boundary layer. To investigate the frequency characteristics of a time domain waveform such as that present in FM temperature control, it is customary to convert the waveform into the frequency domain by means of a Fourier Transform. F (0)) = ft (t) e "ia)t dt - oo For a rectangular wave form of amplitude A and duration T, the amplitude of the Fourier Transform is: F (W) = 2A (A) o4« Nt sin --- 7-8 |