Research On Flow Characteristics And Heat Transfer Mechanism For Backward-facing Step In Transition Flow

In the context of the current international society to cope with the energy shortage and to improve energy efficiency, many heat exchangers widely used as industrial heat transfer equipment are developed towards to the direction of high efficiency and miniaturization. When the tube heat exchange equipment miniaturization, the pipe flow field will appear more transition flow state. The transition flow is a special flow state which is different from the laminar flow and turbulent flow. The heat transfer mechanism of the turbulent flow is not fully applicable to the transition flow. So the study of the heat transfer mechanism of transition flow has attracted the attention of many scholars.The backward-facing step is one of the commonly geometric models to study the separation flow and heat transfer enhancement. There are two major reasons. Firstly, it is a simple geometric model and the flow field shows the complex characteristics of the separation flow. Secondly, when the fluid flows through the sudden enlargement cross section, the flow separation can make the steady flow state turn into the unstable flow state. It is one of the common methods for heat transfer enhancement under low Reynolds number range. The flow field of many high-performance tube heat exchangers has the characteristics of separation flow. So the flow and heat transfer model of tube heat exchanger can be simplified as a backward-facing step model.The purpose of this paper is to interpret the mechanism of heat transfer for the separation flow in the transitional flow. So the flow characteristics and heat transfer characteristics of the backward-facing step in the transition flow is studied, and the heat transfer mechanism is analyzed according to the change of the flow field structure. In the transition flow area, the flow state is complicated and the control equations are nonlinear. The flow and heat transfer numerical model for the backward-facing step is established by the unsteady method in this paper. The flow characteristics and heat transfer characteristics are systematically studied through the numerical simulation, and some achievements with academic significance and practical value have been acquired:The fluid inlet velocity and the geometric parameters are the basic parameters which influence the flow field and heat transfer characteristics. The influences of Reynolds number, step height, channel height and expansion ratio on the Nusselt number at the bottom wall surface downstream of the step are systematically discussed for the low Reynolds number range. The law of heat transfer is analyzed according to the change of the flow field structure. The research for the flow structure is in preparation for the unsteady flow field and heat transfer characteristics in the transitional flow.In the transition flow, the changes of flow field and temperature field with the Reynolds number increase are studied through numerical simulation. The flow field is divided into three conditions according to the recirculation region structure upstream of the reattachment point, then the flow field structure and flow characteristics are further researched. The time average heat transfer at the bottom wall surface is analyzed considering the change of the flow field structure. The influences of temperature, velocity, pressure on the heat transfer characteristics in the different working conditions are compared. The large scale vortex pairs rotate in opposite directions in the middle area downstream of the step and the small scale vortexes are attached to both upper and lower walls in the transition flow. The main peak and the minor peak appear along the flow direction for the local time average Nusselt number distributed on the bottom wall. The heat transfer upstream of the main peak position is strengthened due to the attached vortexes appear inside of the primary recirculation zone.The flow parameters of the backward-facing step change with time and the flow field shows the unsteady characteristics. The local instantaneous Nusselt number and the local time average Nusselt number, the local instantaneous surface friction coefficient and the local time average surface friction coefficient distributed on the bottom wall are compared for different Reynolds number. The instantaneous heat transfer characteristics are analyzed detailedly combining with the instantaneous flow field structure. The distribution characteristics of local Nusselt number and local surface friction coefficient at the same time are compared. According to the flow characteristics and heat transfer characteristics, the unsteady flow area is divided into "the strong periodic areas" and "the weak periodic areas". The bigger the Reynolds number is, the greater the weak periodic area expands and the stronger the unsteady flow field characteristic is.In transition flow, the effect of expansion ratio on the flow field and heat transfer is studied. When expansion ratio becomes larger, the vortexes rotated in opposite directions cut off each other, so the interaction between two adjacent rows of vortexes is intensity. The main peak value of local time average Nusselt number increases, but the value downstream of main peak position is generally low. When expansion ratio decreases, the interaction between positive vortexes and negative vortexes becomes weak. The main peak value of local time average Nusselt number decreases, but the value downstream of main peak position increases. The distance between the main peak and the minor peak increases and the double peak phenomenon subdues. The flow field and heat transfer characteristics change with time are researched for different expansion ratios in the transitional flow. When the expansion ratio decreases, the fluctuation amplitude for local instantaneous Nusselt number decreases. The distance between two adjacent peaks increases indicates the distance between two adjacent attached vortexes increases and the number of attached vortex is reduced within a certain distance.In transition flow, the heat transfer mechanism for backward-facing step is researched combined with the analysis of flow field. The cold fluid in the mainstream is brought into the wall by the negative vortex. The heat transfer effect between cold fluid and hot wall is sufficient and the local instantaneous Nusselt number is improved. However, the velocity is accelerated by the positive vortex near the wall, but its direction parallel to the wall where the heat transfer effect is deteriorated for the temperature boundary layer thickening rapidly and the local instantaneous Nusselt number reduces. The peaks of local instantaneous Nusselt number are near the inflection points downstream of the every attached vortex. The valleys of local instantaneous Nusselt number are always located under the positive vortexes. The vortexes move downstream with time change. The positions of peak and valley of local instantaneous Nusselt number also move downstream with time change.This work can enrich the heat transfer theory for the transition flow. It provides some reference to improve the structure of the heat exchange equipment, then to further enhance heat exchanger efficiency. So it has an important significance in scientific theory and engineering application value.