Numerical And Experimental Study Of Shearing Interferometric Temperature Measurement For Natural Convection Heat Transfer About Horizontal Cylinders

The measurement of temperature field is of great significance for scientific research and national economic development. The regular contact temperature measurement technology is inevitable to destroy the original temperature field distribution because of the intervention of the probe. The optical methods with the advantages of non-contact measurement, high sensitivity, all-field measurement, high precision get a lot of use.It is increasingly important to strengthen the study of natural convection phenomena for the closely relationship between natural convection and our daily life. The natural convection of horizontal heating circular cylinder in infinite space has been used in many industrial applications. For the study of natural convection around horizontal circular cylinder, three mainly kinds of method are adopted. The first is experimental method; the second is to directly solve the N-S equations and numerical solutions of energy equation; the third is to use various approximate method for solving analytical solution of boundary layer equations. In traditional research, the hypothesis of one-dimensional infinitely long is commonly used to ignore the end effect of horizontal circular cylinder, which does not meet the practical industrial applications. When the horizontal circular cylinder is too long or the wall temperature is relatively high, the interference fringes near wall is so dense that sufficient resolution can not be provided. The analytical solutions obtained by using all sorts of approximate method to solve boundary layer equations have differences because the existing plume area at upper part of cylinder does not meet the boundary layer hypothesis.In this paper, the testing principles of various types of optical methods and its applications in heat transfer have been introduced. The characteristics and principles of temperature measurement using ordinary optical interferometry, holographic interferometry and lateral shearing interferometry(LSI) are compared. LSI belongs to common path interferometry and the reference wave surface is unnecessary. LSI is not sensitive to the vibration of machine and the disturbance of the environment. It has simple structure and the fringes obtained using LSI is similar to the results of holographic interferometry and the real-time measurements can be obtained. For the two wavefronts making interference fringes in their overlap area the phase difference of some position can not directly be obtained and it is necessary to make phase reconstruction based on existing interference phase as the shearing quantity is small. When the shearing quantity is large the wavefront reconstruction is not necessary. The similar simple real-time fringes using LSI with large shearing quantity is similar to the fringes using Mach-Zehnder interferometry.The natural convection around finite long horizontal circular cylinder with active ends was experimentally investigated. The real-time fringes of horizontal circular cylinder with different wall temperature were obtained using LSI with large shearing quantity. The effective range of temperature field effected by buoyancy was spread to very high distance. The end effect correction was carried out and within certain temperature range the coefficient a was given. The temperature distribution of some given line after end effect correction were provided. The temperature inversion error was less than 1K usingα=0.88 for end effect correction when wall temperature of the circular cylinder with active ends was between 30℃and 95℃.It is more difficult to solve natural convection because the solving of velocity field and temperature field are coupled with each other. The boundary layer assumptions or Boussinesq assumptions are widely used to solve natural convection. In this paper, the numerical simulation of natural convection around horizontal circular cylinder in infinite space using incompressible ideal gas and Boussinesq approximation model was carried out. The shearing interference fringes using above temperature data and pure background fringes existing defocus and aberration were obtained. The obtained simulation fringes by overlapping the shearing interference fringes and pure background fringes was very similar to the experimental fringes.When wall temperature of horizontal circular cylinder is high to a certain extent, the interference fringes will be so dense that instrument can not distinguish, so most wall temperature is less than 100℃. The real-time fringes of electric heating circular cylinder with insulation ends during 450℃and temperature decreasing to ambient temperature processes were obtained using LSI with large shearing quantity. The quantity of fringes above cylinders is more than below cylinders. With the wall temperature increased, the trend was more evident. The temperature inversion of cylinders with different wall temperature was completed, taking into account end effects. The results showed that end effects was small with both adiabatic ends. The outer end effects was greatly enhanced and the corresponding additional quantity was large with both non-adiabatic ends.