Investigation on Convection Heat Transfer Augment in Spirally Corrugated Pipe

1. Introduction

Heat exchangers are essential energy conversion equipment in industrial fields. It is of practical significance to develop a new type of heat exchanger using a highly efficient heat transfer enhancement technique for promoting energy conservation and emission reduction and achieving carbon neutrality in China. Changing the shape of the heat exchange surface or attaching the destabilization element around the heat transmission surface is the ordinary approach to developing heat delivery elements, such as the spirally corrugated pipe [ 1 ], finned pipe [ 2 ], twisted elliptic pipe [ 3 5 ], and dimpled pipe [ 6 ], which all belong to this category. Spirally corrugated tubes can effectively enhance heat transport with a relatively small flow resistance [ 7 ], and they are easy to manufacture. Therefore, corrugated tubes are used extensively to enhance the heat transfer of shell and tube heat exchangers. Scholars have conducted extensive research on SCPs from the aspects of the inside flow and heat transfer characteristics [ 8 15 ], different working fluids [ 16 22 ], structure optimization and improvement [ 23 25 ], and development of novel heat exchangers [ 26 31 ].

Hærvig et al. [ 8 ] numerically elaborated upon the heat transmission characteristics in 28 SCPs and observed that a further increase in thread height would result in a small elevation in convection heat transmission but a prominently larger flow drag for the cases of high thread height. Hido et al. [ 9 ] evaluated the friction loss and enhancement in heat transfer coefficients experimentally and found the optimal tube geometry parameters. Based on experimental verification, Córcoles-Tendero et al. [ 10 ] inquired into the convective heat transmission characteristics in SCPs by means of numerical simulation and obtained relatively accurate simulation results. Qin et al. [ 11 ] applied the particle image velocimetry system to measure the instantaneous flow field inside SCPs. They found that the SCP could generate multiple longitudinal vortices to accelerate convection heat transmission inside the pipe in the absence of external heat transfer enhancement technology. Zhang et al. [ 12 ] concluded that the vortex between two adjacent corrugated grooves was the root cause of the heat delivery improvement in the corrugated pipes. Al-Obaidi and Alhamid [ 13 15 ] numerically investigated the thermohydraulic performance of SCPs; they examined the flow behavior, friction resistance, and heat transport performance inside SCPs with varying geometrical configurations and developed the correlations for heat transport performance calculation.

In view of the different physicochemical properties of various working fluids on the convection heat transmission performance, some researchers analyzed the convection heat transmission characteristics of different working fluids flowing through SCPs. Vicente et al. [ 16 ] experimentally researched the tube side flow behavior and convection heat transmission performance for SCPs with water and ethylene glycol as a working medium. In addition, a dimensionless parameter associated with the flow and convection heat transmission characteristics was defined, and the correlated formulas for the calculation of the flow resistance and convection heat transmission performance were obtained. Yang et al. [ 17 ] examined the frictional loss and convection heat transmission performance of four SCPs with different geometric parameters by utilizing oil and water. The results revealed that SCPs increased the convection heat transmission performance by 0.3~1.2 times in the case of increasing the friction coefficient of the flow resistance by 0.3~1.6 times in contrast to the plain pipe (PP). Rozzi et al. [ 18 ] took fluid foods as a medium to examine the convection heat transport and flow drag characteristics in SCPs. Some researchers also used the SCPs in refrigeration devices to develop efficient evaporators or condensers. It is very important to clarify the boiling and condensation heat transfer characteristics of refrigerant in the SCPs. Laohalertdecha and Wongwises [ 19 ] concluded that the thread pitch affected the convection heat transmission performance promotion and flow drag enlargement significantly through experiments with R-134a. Aroonrat and Wongwises [ 20 ] researched the thermal–hydraulic performance improvement with the usage of corrugated tubes for evaporating flow conditions. Li and Tang [ 21 ] developed a simulation model coupled with a microchannel heat exchanger model to explore the drop-in alternative mixtures for R134a regarding a mobile air conditioner. Li and Hrnjak [ 22 ] presented the phase separation characteristics in the vertical second header of a condenser through a computational fluid dynamics simulation. The results showed that the liquid separation efficiency decreased as the vapor separation efficiency increased, following a linear trend in the experimental range.

The application of SCPs can be considered as an efficiently passive enhancement method to upgrade heat exchangers, and heat exchangers composed of SCPs have been continuously improved since their development. A double tube heat exchanger made up of SCPs, instead of PPs, could improve the convection heat transmission performance by about 1.73 times [ 23 ]. Qian et al. [ 24 ] improved the convection heat transmission performance of SCPs by increasing the number of threads and assessed the optimization results with the performance evaluation of comprehensive convection heat transfer. Yang et al. [ 25 ] identified the influencing factors including the head number and ripple depth and space on convection heat transmission performance using the Taguchi method and obtained the optimal structure to upgrade heat transmission performance. With the gradual revelation of the heat transmission intensification mechanism and the continuous optimization of the geometric structure of SCPs, the heat exchange equipment applying SCPs as the core heat transfer units are emerging in an endless stream. Liu et al. [ 26 ] proposed a longitudinal flow baffle rod heat exchanger using SCPs with different spiral heads and researched the shell-side thermohydraulic performance. The results implied that the overall heat transmission performance of SCPs with single heads is the best. With the external expansion of SCPs, Wang et al. [ 27 ] proposed a SCP tube and shell heat exchanger and analyzed the influence of the pipe bundle arrangement on the thermohydraulic characteristics of the heat exchanger. Wu et al. [ 28 ] and Moya-Rico et al. [ 29 ] examined the heat exchange performance of a concentric pipe heat exchanger composed of SCPs and explored the effect of the physical parameters of SCPs on the heat exchanger performance. Hu et al. [ 30 ] used three corrugated pipes to improve the intermediate heat exchanger performance and investigated its thermohydraulic characteristics. Pethkool et al. [ 31 ] inquired into the convection heat delivery improvement in a concentric pipe heat exchanger made up of SCPs with water at a turbulent state.

Scholars have carried out abundant research on the fluid flow and convection heat transmission performance in SCPs in terms of structural optimization and application. Many studies reported that the heat transfer performance of SCPs was better than that of PTs. Is there any basic reason to reveal this phenomenon? How to optimize the tube type of SCPs? Is there a better design scheme for the heat exchangers developed with SCPs? The premise to answer these questions is to clarify the mechanism of the heat transfer enhancement of SCPs. However, past studies have reported on the heat transfer enhancement effect more than the analysis of the heat transfer enhancement mechanism. Clarification on the heat transfer enhancement mechanism of SCPs is the basis for realizing effective heat transfer enhancement and optimizing the tube shape. Meanwhile, the geometric shape of the SCP is inseparable from its heat transfer augmentation effect. In our opinion, the geometric structure is an external factor, which induces the spiral flow inside the pipe, then the spiral flow acts on the boundary layer to reduce the thickness of the boundary laminar flow. Therefore, the relationship between the geometric structure size and the spiral flow and the improvement mechanism of convection heat transmission in SCPs needs to be explained in depth.

Based on the existing experimental data validation, the paper examines the convection heat transport and flow drag performance of SCPs through numerical calculations. The variations of the convection heat transmission and flow drag performance of SCPs with their geometric parameters and the flow parameter are explored. In addition to the external factor of heat transfer enhancement, the root causes of convection heat transfer improvement in SCPs are provided. Then, the elevation of the convection heat transmission performance of SCPs is evaluated based on the comprehensive heat transmission performance.