Heat Transfer Characteristics of Tube in Heat Exchanger

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Environmental issues which include global warming and rapid climate change have become a serious problem faced by us today. This occurs when certain gases such as carbon dioxide (CO₂), Nitrous oxide (N₂O), chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) in the atmosphere traps heat radiating from Earth toward space, which is also known as the greenhouse effect. To tackle this problem, energy conservation is considered an important field of study and many researchers have done researches on the ways to improve energy conservation.

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Renewable energy such as solar energy is widely used by humans today because it is the most common and sustainable renewable energy. Solar energy is used for domestic heating but some problems such as instability and intermittency are faced during the utilization of solar energy. So, research on energy storage and conservation is being conducted to overcome this problem.

Phase change materials (PCM) should be used in the thermal energy storage system because it can address the intermittency of the solar resource. Most of the studies reported the single-channel heat exchanger, but it is not applicable for shell-and-tube heat exchanger (STHE) . Yuxin Zheng and Zhihua Wang (2018) have conducted a study on the heat transfer characteristic of a shell-and-tube phase change energy storage heat exchanger by integrating PCM with the thermal performance of STHE. From their studies, they found that the natural convection effect of PCMs on the heat exchange is more intense in STHE than in single tube during the cooling process and this heat transfer mode in the whole process involves mainly heat convection and conduction. However, the melting blind zone will reduce the melting efficiency because nearly 70% of the PCM is melted through natural convection.

This is mainly due to the flow up and scour of the liquid paraffin wax during the natural convection of the liquid phase material in the early and middle melting stages. However, the gravity at the bottom of the tank is not enough to form convection in the process of thermal energy storage, so the heat conduction is the main heat transfer mode. This is the main weakness of the low energy storage efficiency of heat exchangers because there is almost no melting phenomenon at the bottom of the heat exchanger.

Moreover, the main reason that caused global warming and ozone layer depletion is the rapid use of CFC and HCFC in conventional refrigeration and air conditioning systems. However, the use of CFC and HCFC has been restricted and CO₂ is encouraged to be used as it has zero ozone depletion potential. CO₂ has attractive thermodynamic and physical properties on specific heat at constant pressure and low specific volume. According to Crespi et al (2017), he studied that the supercritical pressure CO₂ Brayton cycle can perform well with high-temperature heat sources and it is considered as a highly efficient power generation cycle. Thus, this method can become a new option for power generation in the future.

Y.H. Zhu et al have conducted a study on the characteristics of the flow and heat transfer of supercritical pressure CO₂ in two fluted tube-in-tube heat exchangers under different flow rates and pressures. Throughout the experiments and the result they get, they conclude that when the bulk CO₂ temperature reaches a maximum pseudo-critical temperature condition, both the local heat exchange rate and the heat transfer coefficient will increase. Other than that, they also found that the effects of both the fluid properties and the fluted tube structure should be considered during the cooling process of the convective heat transfer correlation of supercritical CO 2 in the fluted tube because the structural factors will have an influence on the reduction of overall dimensions, increase of thermal efficiency and reduction of hydraulic losses for the tube in heat exchanger. Besides, most heat exchangers use shell-and-tube heat exchangers as they can withstand high temperature and pressure. Valery Gorobetsa et al have made an investigation on the heat transfer and hydrodynamics of heat exchangers with the compact arrangement of tubes. After they made some experiment and analysis, they concluded that the structure of the compact tube developed are sufficiently effective at a significant reduction in the mass-dimensions of the heat-exchange surface. It is shown that for a bundle with a compact configuration, the local heat coefficients is approximately 2 times greater compared to inline tube bundles, which means the total heat transfer coefficient on the surface of the bundle of new construction will increase as well. This is because of the short length of the formation of a boundary layer on the tube surface in compact beams and a large number of such areas per unit length of the channel in comparison with the traditional tube bundles. The new design of heat exchangers with the compact tube bundles is proposed, which has high efficiency, low aerodynamic and hydraulic resistance. The heat exchangers of new construction have dimensions of 1.7–2 times smaller and mass is 10–15% lower compared to heat exchangers of traditional designs with the same heat power.


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