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The review of the articles presents the characteristics of heat transfer in tube. Heat transfer of fluid and refrigerant flowing through a tube can occur in different types of thermal processes. Besides that, the design of tube can lead to other factors of heat transfer. For examples, heat exchangers, condensers and evaporators. Therefore, it is important to know heat transfer coefficient in this situation.
Energy can neither be created nor destroyed but can be transferred from one system to another. The transfer of heat is one of the major forms of energy transfer whether in day to day activities or even in an industrial perspective. The transfer of heat can be done through many methods a few examples of which are; thermal conduction, convection and radiation. Thermal conduction is the transfer of kinetic energy at a microscopic level through the walls of the system. Thermal convection is when a when a flow of liquid or gas carries heat along with the flow of matter. Thermal radiation is the transfer of energy through photons in electromagnetic waves.
Heat transfer can be carried out through tubes. The characteristics of heat transfer in tubes is a major factor to consider during the selection of the tubes. There are multiple types of tubes and each has its own individual properties that can either enhance or reduce the transfer of heat. The selection of the type of tube purely depends on the purpose that it is intended for. Heat exchanger for example require tube of specific dimensions, enhancing the tubes in a two-phase flow system further improves the performance of the device and can help boost development of high-performance air conditioning and refrigeration systems.
During the flow of fluid or gas in the tubes, a small portion tends to evaporate, this leads the system to have a vapor liquid two phased flow as mentioned above. One of the few factors that affect the condensation heat transfer coefficient are mass flow rate along with vapor quality, the saturation temperature and the difference in temperature. The understanding of the benefits in each method of heat transfer helps us analysis the characteristics of the heat transfer in the tubes.
Swirling Flow Heat Transfer.
The melting point of the metal materials must be far higher than the temperature in the turbine to improve efficiency of a gas turbine and prevent the materials from melting. Cooling system of the turbine must be highly efficient to extend the lifespan of the turbine. Conferential swirling flow transfers heat in a very efficient manner. It also distributes the heat transfer uniformly. Factors such as jet number, working medium, spacing and angle, tube shape, Reynolds number and temperature ratio have influence on the efficiency of heat transfer.
The idea of swirl cooling was brought up by Kreith and Margolis. High axial velocity near the wall and the enhanced turbulent mixing are the major mechanisms leading to high heat transfer.
Heat Transfer in a corrugated tube
Heat transfer coefficient compensates the increase of pressure drop induced by the corrugation in a corrugation tube. The advantages of corrugation tube are that the material used to produce a corrugated tube is not more than that used in the production of smooth tubes and. In addition, the manufacturing is not more complex than that of other enhancement tube technique.
Microalgae has high growth rate and high caloric value. Due to these advantages of microalgae, it has drawn the attention of microalgae biomass for biofuel production. It performs photosynthesis and absorbs carbon dioxide. It can also produce and store macromolecular organic matter (e.g., carbohydrates, lipids, and proteins). In order to convert microalgae biomass to biogas, anaerobic digestion is carried out. Biochemical conversion is a temperate and low-energy consumption energy used for this purpose. One disadvantage of microalgae cells is the dense structure hinders the digestions of microalgae by microorganism. Hydrothermal pre-treatment is needed to loosen up the structure. Convective heat transfer characteristics requires four important thermophysical parameters that are specific heat capacity, density, thermal conductivity, and viscosity of the microalgae.
Hydrofluoroolefins (HFOs) such as HFO-1234yf or R1234yf are used as replacement for R134a in conditioning system. The efficiency of the cooling system for 1234yf has not been fully tested and comprehensive. Lee and Jung carried out a test to determine the efficiency of the R1234yf as refrigerant and compare the results to that of R134a. When compared, a decrease of approximately 7% in cooling capacity and 4.5% in COP was found in R1234yf systems without internal heat exchangers. R1234yf refrigeration efficiency can be improved by ambient subcooling, external heat exchangers, subcooling with liquid-suction heat exchangers and mechanical subcooling. The work of Qureshi and Zubair explains the each of the methods.
Heat transfer characteristics of tube in heat exchanger.
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) or chlorofluorocarbon (CFC) 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.
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.
Schematic diagram of the tube-and-shell heat exchanger
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 hydrochlorofluorocarbon (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 are 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.
Process of heat transfer in finned heat transfer tube.
Finned heat transfer tube was cooled by air under the low flow rate condition for example like natural convection. The usage of finned tube is associated with heat exchange. The design of finned tube can be found in some sophisticated machine.A decay heat removal system (DHRS) for light water reactors (LWRs) was using air cooled finned heat transfer. The system was design with 6 finned hear transfer tubes. The water was added as the medium to transfer the heat out of the system. The system was introduced by Mochizuki and Yano in2015. The picture below shown the DHRS system.
The system was to design to aid cooling in the nuclear reactor core. The picture below shows the picture of finned heat transfer tube and air cooler.
The finned heat transfer tube (HTTs) of the AC cooled the heat transfer between liquid sodium and the secondary system. There is a capacitor motor maintaining a constant flow rate. The flow rate is usually in the range of 10%. The air cooler is place higher elevation and maintain the temperature of the circulation head. The flow rate control unit will control the flow rate to prevent from overcooling of the liquid sodium. The sodium will solidify at 100 degree Celsius.
Effect of tube shape on heat transfer characteristics.
Horizontal tube falling film evaporator has a lot of advantages. One of the advantages was high heat transfer coefficient and low heat transfer temperature differences. This design was widely use in the heavy industry. The manufacturing process for this type of tube was complex. To get the best heat transfer for the tube the shape has play a major part when come to design the tube. Two tube shape are researched by researcher, The flat tube shape and the circular tube shape. In the article researcher state 1 circular shape and 4 different circumferential dimensionless location flat tube shape. The heat transfer coefficient is a significant parameter for measuring the heat transfer. The heat transfer coefficient decreases as the circumferential dimensionless location decreases. As shown in the picture below.
When R number increases from 0 to 2, heat transfer coefficient gets larger. When compare both tubes shape the flat tube shape has a better heat transfer performance. For the flat tube heat transfer performance are usually around 2.2%, 4.2% and 11,.2% which is higher than circular shape. One of the reasons was explained the flat tube shape has higher velocity and thinner film. The result difference between this four-shape tube in the article was concentrate in upper part of the tube. The thickness distribution was similar between the tube shape. For the upper half part of the tube is relatively large and heat transfer in this area go up as well. The component force of gravity of flat tube in flow direction is better when comparing the circular tube shape. Therefore, the increase of R number. The flat shape tube 4 has a higher velocity and heat transfer than the other flat shape tube. The flat shape 4 has the larger circumference are the factor in this case. In conclusion the Flat shape are better in heat transfer than circular shape tube was due to higher flow rate which is higher velocity when transfer the air and thinner wall. Picture below are Dimensionless temperature versus circumferential dimensionless location of the film thickness at different tube shapes.
Study on heat transfer and flow characteristics in the shell side of helically coiled trilobal heat exchange.
The helically coiled tube heat exchanger was called as HCCT heat exchanger was purposed.  It was a heat exchanger capable of obtaining greater heat transfer despite of small temperature differences by its compact structure. Due to small size it is cost effective. This design was adapted by lost manufacturer due to the advantages it has. The application has been found in food, chemical, metallurgical etc. The study of this HCCT structure on flow characteristic and heat transfer was carried out. There are three type of tube, Helically coiled trilobal tube (HCTT), Helically coiled elliptical tube (HCET) and Helically coiled plain tube (HCPT). Temperature difference for the inlet and outlet of HCET, Nusselt number and friction factor at shell side are decreases when Reynold number increases. Reynolds number is a dimensionless value for determine whether the fluid is laminar flow or turbulent flow. The pressure will increase while the Reynold number increase. All the parameter above for HCET was smaller than HCPT when tested together with HCET. The explanation was HCET was consistent with the direction of flow and suppresses the resistant of the shell side flow channel and back vortex. For HCTT shell side the increase of Reynold number was increase while the Nusselt number and friction factor was decrease. For the pressure, the Reynold number was while the pressure drops. All the HCTT parameters are larger than HCPT. For HCTT has the best performance from all the parameters when compared to HCPT and HCET. HCET shell side radical flow velocity was small. The convective heat transfer rate is related to synergistic angle. The magnitude of velocity and temperature gradient was indicated the HCTT shell side radial flow velocity of HCTT is largest.
Single-phase and two-phase heat transfer for hydrocarbon refrigerants inside a helical tube.
The experiment was carried out by the researcher to find single-phase and two-phase heat transfer for hydrocarbon refrigerants inside a helical tube. The experiment set up are as shown by picture below.
The test result was to obtain the local heat transfer coefficients and two-phase flow pattern for hydrocarbon refrigerant in helical tube. The condensation heat transfer coefficient was relying on the flow pattern. The flow pattern gives us an idea of how heat and momentum during the process. The flow pattern was obtaining by using advanced high-speed camera. The annular flow pattern and non-annular flow pattern was captured. The pattern can be calculated with complex calculation. The calculation was related to vapor quality and mass flux. The heat transfer coefficient was increase while the increase of vapor quality and mass. The coefficient will decrease while the saturation pressure increase. The different flow pattern was observed such as annular, wavy, transition, stratified and slug flow were proposed for the methane or propane hydrocarbon mixtures refrigerant for helical tube in condensation. For the experimental heat transfer coefficient simulation can be compared with exiting simulation. The comparison can predict the annular flow pattern and the mean absolute deviation MAE.
Having analysed the different types of tubes and their characteristics during the heat transfer process we were able to conclude on a few important findings. The greenhouse gas effect caused by radiation from the surface of the planet is one of the main reasons for environmental problems as these gases rise to the atmosphere and causes holes in our ozone layer. The production of these greenhouses’ gases are from day to day processes that we do and also from the heat transfers processes in some case. Having understood this, the conservation of energy is considered an important field of study as many pieces of research are being conducted to find ways to improve the conservation of energy. From the results above, we were able to understand that the finned heat transfer tube was one of the best tubes to reduce the loss of heat to the environment during the heat transfer process. Finned heat transfer tubes have a flow rate usually in the range of 10%. We were also able to conclude that the flow rate in the tube is directly proportional to the transfer of heat, the lower the flow rate, the lower the rate of heat transfer. Horizontal tube falling film evaporators are found to have high heat transfer coefficient and low heat transfer temperature differences and this, in turn, increases the overall efficiency of the energy transfer. The overall purpose behind fining different tubes and methods of heat transfer is for the improvement of our daily lives. These tubes are being used in devices and products that we use on a daily basis, for example, air conditioners, refrigerators, heat pumps, etc. The researches that have been conducted before, that are being conducted now, and will be conducted in the future is to increase the efficiency of the current technology of tubes by improving the current designs or by making new designs. This can help not only by bettering our lives, but also helping our environment making a cleaner and greener place to live in.