Study On Fluid Flow And Heat Transfer Characteristic In Half Coil Jackets

For the virtues of high reat transfer rate, high bearing capacity and compact structure, half coil jackets are widely applied in many kinds of industries. Fluid flow and heat transfer characteristic in the outer and inner half coil jackets were studied by numerical and experimental methods. The purpose of the paper is to provide theoretical basis for the design of half coil jackets in practical engineering, thus to reduce flow resistance and enhance heat transfer ability of fluid in the two types of half coil jackets.Based on orthogonal helical coordinate, the mathematic models were setup for the fully developed laminar flow and heat transfer in the two types of half coil jackets. Then fully developed laminar flow and temperature fields were solved with the program self-compiled. The effects of dimensionless axial pressure gradient Ps, curvature ratio, dimensionless pitch of jackets and Prandtl number of the heat transferring fluid on the developed laminar flow and heat transfer characteristic were studied. Results show that the position of maximum axial velocity is only one and the pattern of secondary flow is two-vortex on the cross section of the outer half coil jacket. However in the inner half coil jacket is found to exhibit a transition of secondary flow pattern from two vortices to four vortices when the Dn number increases and the critical Dn value is 96. At the same condition, laminar heat transfer ability of the inner half coil jacket is much higher than that of the outer half coil jacket.Flow fields of the two types of half coil jackets with finite pitch were measured with three dimensional laser-Doppler velometer. Distributions of three dimensional time-averaged and pulsatile velocities in rectangular coordinate were given. By coordinate transformation, the distribution of axial velocity and secondary flow pattern were displayed based on orthogonal helical coordinate.With CFD software FLUENT, fluid turbulent flow and heat transfer in the two types of half coil jackets were simulated. Good comparisons were found between the flow fields obtained with LDV and the results of heat transfer experiment. Based on the simulated results, fully developed turbulent flow resistance fRe and average Nusselt number Num on the heated wall of the two jackets were fitted. The results show that at fully developed turbulent flow condition, secondary flow pattern of the outer half coil jacket is constant two-vortex but of the inner half coil jacket is constant four-vortex. Turbulent flow couples with secondary flow take effect on turbulent convective heat transfer in the two types of half coil jacket. At the same condition, the inner half coil jacket is superior to the outer half coil jacket in turbulent heat transfer ability, but the turbulent flow resistant of the inner half coil jacket is much greater than that of the outer half coil jacket.Based on the conclusion that the centers of the heated wall in the two types of half coil jackets are the poorest positions for heat transfer, the methods of placing fin and pin on the centers of the heated wall are put forward to further enhance heat transfer ability of the outer half coil jackets. Effects of the length of fins and the height of installed pins on heat transfer enhancement were studied. Results show that in the scope of research, fins of h=0.5a length is best for heat transfer enhancement of fully developed laminar flow and fins of h=0.25a length is best for overall heat transfer enhancement in the outer half coil jacket. Pins of h₁=0.5a height is better than that of h₁=a height for overall turbulent heat transfer enhancement in the outer half coil jacket.