Twist in time laminar flow10/5/2023 ![]() ![]() The solution of energy equation generally adopts the energy integral equation or use similarity variables to reduce the mathematical difficulties. These studies have been made for the case in which the velocity field is independent of time. Cess (1961), Riley (1963), and Cotta and Özişik (1986) analytically studied the thermal response of an unsteady laminar boundary layer on a flat plate due to step changes in wall temperature and wall heat flux. ![]() ![]() Few studies have focused on the twisted plate itself.įorced convection process with transient heating boundary is much more complex than the UWT or UHF boundary conditions due to the thermal response behavior in the thermal boundary layer. Most of the works deal with tubes or ducts with twisted tape as inserts and study the heat transfer enhancement effect with uniform wall temperature (UWT) or UHF boundary conditions. (2019) compared the thermal performance of a circular tube with twisted tapes, perforated twisted tapes, and perforated twisted tapes with alternate axis. (2018) studied the effect of twisted rib over the absorber plate of solar air heater with a maximum enhancement for heat propagation and friction factor to be 2.58 and 1.78 times that of smooth surface, respectively. They suggest that this shape of twisted tape has an advantage in heat transfer enhancement over the typical twisted tapes. (2017) experimentally studied the heat transfer and friction characteristics of dual-pipe heat exchanger with alternation of clockwise and counterclockwise twisted tape. (2001) conducted experimental study for regularly spaced twisted tape–induced laminar tube flow with uniform heat flux (UHF), and the effect of tape width was discussed. Manglik and Bergles (1993a) and Manglik and Bergles (1993b) obtained empirical correlations from wide experimental conditions for isothermal tubes with twisted tape as inserts. Thus, many research studies focused on the flow, and heat transfer characteristics for tube with twisted inserts have been carried out. Among all these techniques, twisted tape can change the flow pattern in a channel, reduce thickness of the boundary layer, and increase the heat transfer area. Detailed surveys of enhancement techniques have been made by Patil and Farkade (2016), Menni et al. Researchers have been committed to increase heat transfer coefficients in all types of heat exchange equipment. Heat transfer enhancement technology is of great importance in industries, such as power generation industry, chemical industry, and automotive. It is considered that, when the penetration depth is smaller than the thermal boundary thickness, the heat transfer from the interface (wall surface) to the fluid domain is not fully developed during the disturbance. The response analysis for transient heat transfer at fluid-solid interface was conducted by applying the concept of penetration depth. The heat transfer coefficient is found to increase with the decrease of τ, and the transition point was estimated to be at τ ≈ 1 s, which means that, when the increasing ratio of heat generation rate satisfies d Q d t ≥ Q 0 ⋅ e t, the heat transfer enhancement phenomenon will be observed. Helium gas with inlet temperature of 298 K under 500 kPa was used as the coolant. A platinum plate with five pitches (each was 180° twisted with 20 mm in length) was used in the experiment. A wide range of τ from 37 ms to 14 s was applied for the experimental study. The twisted structure was heated according to exponential function ( Q= Q 0 ×exp ( t / τ ), where Q 0 is the initial heat generation rate, W/m 3 t is time, s and τ is the period of heat generation rate). This study was conducted to investigate the transient heat transfer characteristics of a twisted structure. School of Mechanical Engineering, Nantong University, Nantong, China.
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