Research On Heat Transfer And Fouling Cleaning Enhancement Of Pulsating Flow And Tube Vibration

The performance of the heat exchanger can be improved by enhancing heat transfer and reducing fouling resistance. A lot of researches have been done on flow field and convection heat transfer with pulsating flow, the results is not consistent, but the flow pulsation can reduce the fouling resistance. Furthermore, when the flow across the heat exchanger components, the flow pulsation and pressure pulsation are produced, the tube vibrates induced by the flow, and then the heat transfer is enhanced.The influences of the pulsating flow on the fouling cleaning in channel and tube, the effects on the flow and heat transfer characteristics for pulsating laminar flow in a square cylinder, the heat transfer enhancement induced by the tube vibration are theoretical, experimental and numerical studied.A two-dimensional mathematical model of pulsating flow in channel is established,the wall shear stress and velocity distribution are theoretical analysised based on variable separation method. The effects of pulsating flow parameters on the fouling resistance is analysised by comparing the shear stress for steady flow and pulsating flow.The shear stress for pulsating flow increases linearly as the amplitude increases under low frequency. There is a critical amplitude, which is π. When the amplitude is greater than the π, the wall shear stress ratio is greater than 1, and then the fouling resistance decreases.The energy efficiency of the pulsating flow on fouling cleaning is also discussed, the flow with low frequency and amplitude is more energy efficient. When the input power of the steady flow is equal to the reciprocating flow, only when the dimensionless frequency is greater than 2, the heat transfer enhanced.Furthermore,the distribution function of velocity in the circular tube with laminar pulsating flow is theoretical analysised. Based on the Bessel Function, it is derived that the shear stress near the wall surface has an analytic solution. There is an influence on the shear stress near the wall surface which relates to the frequency and amplitude when providing a pulsating pressure. The result shows that the wall shear stress ratio between pulsating flow and steady flow increase linearly with the increasing of the amplitude when the frequency is low; the stress ratio is close to zero when the frequency is high. And the variety law of the shear stress when the pulsating flow is constant during half cycle has been discussed, The stress ratio can be calculated to be 1 with a critical value of frequency, when the frequency is bigger than the critical value, the shear stress increase with the frequency, and then the fouling cleaning is enhanced.An experimental study is used to analysis the effect of pulsating flow on fouling growth in this paper. A fouling experimental system, including the experimental equipment and measurement system is built for the experiment study. The thermal resistance method is adopted to measure thermal resistance of fouling under pulsating flow. It is found that pulsating flow can reduce the fouling resistance, fouling resistance decreased by 25 % compared with the steady-state when the frequency of pulsating flow is 2Hz, and with the increment of the frequency of pulsating flow, the induce period became longer, the deposition growth rate slowed down, the asymptotic fouling resistance also decreased. With the effect of pulsating flow, the maximum wall shears tress of the tube is larger than value under the steady flow, the denudation rate of fouling was higher, and the faster frequency was, the more obvious the effect is.Two-dimensional numerical simulation is performed to understand the effect of flow pulsation on the flow and heat transfer from a heated square cylinder at Re=100. Numerical calculations are carried out by using a finite volume method based on the pressure-implicit with splitting of operator’s algorithm in a collocated grid. The effects of flow pulsation amplitude and frequency on the detailed kinematics of flow(streamlines, vorticity patterns), the macroscopic parameters(drag coefficient, vortex shedding frequency) and heat transfer enhancement are presented in detail. The lock-on phenomenon is observed for a square cylinder in the present flow pulsation. When the pulsating frequency is within the lock-on regime, time averaged drag coefficient and heat transfer from the square cylinder is substantially augmented, and when the pulsating frequency in about the natural vortex shedding frequency, the heat transfer is also substantially enhanced, the heat transfer performance can be enhanced to 16%. In addition, the influence of the pulsating amplitude on the time averaged drag coefficient, heat transfer enhancement and lock-on occurrence is discussed in detail, From the frequency and length of the trailing vortex recirculation zone directly determines the heat transfer performance is directly determined by the vortex shedding frequency and the the length of the recirculation zone.Finally,the vortex-induced vibration(VIV) and heat transfer of a circular cylinder at low Reynolds number is numerical analyzed. The cylinder motion is modeled by a mass-spring-damper system, and the motion equation of the cylinder is solved by Newmark-β method, the dynamic mesh technique is used to solve the coupling between the cylinder and flow. The results including “Lock-on” and “Beat” phenomena are obtained, and the effect of mass ration on "Lock-on" is analyzed, furthermore, the influence of vortex-induced vibration on convective heat transfer is also discussed. When VIV in a "locked on" state, the vortex shedding frequency from the heat exchanger components is quicker, and the length of the recirculation zone is shorten, then the heat transfer enhanced.