TUGAS BESAR CFD : Heat Transfer Enhancement Using Screw Insert Tapes

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1. Introduction

In general, there are two methods that can be done to increase the heat transfer rate, namely through active and passive (Paneliya et al. 2020). The increase in the heat transfer rate through the active method is carried out by utilizing external energy such as forced convection using the aid of a fan. Meanwhile, increasing heat transfer through passive methods does not require external energy, including by using twisted insert tapes (Paneliya et al. 2020). The use of twisted insert tapes can increase the heat transfer rate of a passive stream. The twisted tapes insert is shaped like a ribbon that is twisted and placed in the middle of the pipe.

Twisted-tape inserts are beginning to be applied in a wide variety of areas due to their ability to improve heat transfer performance at a low cost (Paneliya et al. 2020). Basically, the presence of twisted insert tapes functions as a swirl generator and will modify the flow, especially in areas close to pipe walls (Martemianov and Okulov 2004; Paneliya et al. 2020; Smithberg and Landis 1964; Webb, Narayanamurthy, and Thors 2000). The swirl flow will increase the normal velocity gradient on the pipe and result in a reduction in the thickness of the thermal boundary layer, so that the heat transfer will be higher (Shinde, Patil, and Dange n.d.). Meanwhile, the use of twisted insert tapes has a consequence of increasing pressure drop in the flow and will have an impact on higher energy requirements to overcome this pressure drop (Paneliya et al. 2020). Therefore, increasing the heat transfer rate using twisted insert tapes needs to be considered so as not to produce too high a pressure drop.

Research and studies on the use of twisted insert tapes have been carried out by many researchers. Wang and Sunden, (2002) conducted a study on the heat transfer coefficient through the flow of water flowing in a tube equipped with a twisted insert tape. And it was found that there was a 3x and 3.5x increase in laminar and turbulent flow, respectively, to the water flow in the tube without inserts. Then, Agarwal and Rao, (1996) and Sarma et al., (2005), made a correlation between pressure drop and heat transfer on a tube using a conventional fitting model. From these studies, it was found that the presence of a tape insert would inhibit the transition jump from laminar to turbulent. However, because it causes a continuous fluctuation in turbulent flow, the heat transfer will be drastically decreased.

In addition to using inserts, finally various shapes and configurations to increase the heat transfer rate were studied further, such as twisted tape, corrugated / grooved tubes, and combination of turbulators (Paneliya et al. 2020). Sivashanmugam et al., (2008) conducted a study on the heat transfer and friction factor characteristics of circular tubes equipped with helical screw tape inserts. It was found that the helical twisted tapes produced better performance than the previously reported twisted tapes. Promvonge (2008) conducted an experiment using a wire coil connected to a twisted tape to increase the heat transfer rate and showed that the combination of wire coil and twisted tape produced better heat transfer, compared to only wire coil or twisted tapes. Piriyarungrod et al. (2015) conducted a study on the effect of twisted tapes with the shape of the tapper and the angle of the tapper on the heat transfer rate. From this research, it was found that the twisted tape in the form of a tapper will increase the heat transfer performance, friction loss, and thermal performance factor by increasing the angle of the tapper. He et al. (2018) have also conducted a study on the heat transfer characteristics of tubes equipped with cross hollow twisted tape inserts. However, the mechanism for increasing heat transfer with cross hollow insert tape is quite complex. Sarviya and Fuskele, (2018) conducted an experimental study on increasing heat transfer with circular tubes with a twisted tape insert that has a continuous cut edge. The result is the same: a reduction in the twist ratio will increase the heat transfer rate, but also increase the pressure drop.

After finding the fact that the use of insert tapes can increase the heat transfer rate, several other researchers conducted further studies, namely the effect of variations in the geometry of insert tapes on the heat transfer rate. Bhuiya et al., (2016) conducted an experimental study of several helix-shaped tape inserts that had different pitch ratios and were applied to turbulent flow. This study shows that if the pitch ratio value gets smaller, the heat transfer coefficient and pressure drop will increase. However, the increase in heat transfer is more significant than the increase in pressure drop, so the energy used will be more efficient. Garcia et al., (2018) Studied three different wire coil inserts and three different twisted insert tapes for applications in solar collectors and found that there was an increase in heat transfer and a decrease in temperature on the walls. Then, Yadav et al., (2012) Performed CFD simulations on applications of twisted tapes that have different lengths, namely the full length (from upstream to downstream), only half the length on the upstream side, and only half the length on the upstream side. downstream side. It was found that the full length twisted tape insert provided the highest heat transfer rates. Meanwhile, twisted tape inserts that are only half long on the downstream side have better performance than those that are only half long on the upstream side. Bhuyan et al., (2017) Conducted a numerical investigation regarding the comparison of heat transfer characteristics between twisted tapes inserts with full length and short ones under transient laminar flow conditions in U-loop shaped pipes. It was found that the highest exit temperature was generated in U-loop shaped pipelines with twisted insert tapes of full length and then followed by U-loop pipelines with short twisted insert tapes. Both result in higher outlet temperatures compared to ordinary pipes (plaint tubes). Another CFD simulation study on twisted insert tapes was carried out by Piriyarungrod et al., (2015). This study showed that twisted insert tapes with clearance ratio with value of 0.05 produced the highest performance compared to other clearance ratio values.

Based on a review conducted by Yousif, A.H. and Khudhair M.R. (2018), and also from some literature that the author has explored, until now there has been no study that discusses the use of insert tapes for use in evaporation applications. Referring to the results of studies that have been carried out, it is hoped that the use of insert tapes can also increase the rate of evaporation. This study is intended to find out the use of the screw tape insert as an heat transfer enhancement passive technique for evaporation process.

2.Materials

This study also compares two screw inserts geometry with different pitch length; 25 mm (left) and 25 mm(right). The length of the screw is 100 mm. The inner and outer diameter of the geometry sucessively are 15 mm and 20 mm. Both geometry is placed inside a channel with 120 mm long as shown in Fig.1 below.

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