Melden

AmritanshManthapurwa1Folgen

3. Mar 2023•0 gefällt mir•15 views

3. Mar 2023•0 gefällt mir•15 views

Downloaden Sie, um offline zu lesen

Melden

Wissenschaft

Review of advancements in heat transfer

AmritanshManthapurwa1Folgen

Nbhtc pureNandi Sudheer

Basic Study on Solid-Liquid Phase Change Problem of Ice around Heat Transfer ...IJERDJOURNAL

Vortex Tube Usage in Cooling and Liquification Process of Excess Gases in Ghe...Samet Baykul

International Journal of Computational Engineering Research(IJCER)ijceronline

D031201025031ijceronline

Study on Effect of Twisted Tape Insert Geometry in Heat Exchangerijtsrd

- 1. 1 NAGPUR Recent Advancement in the field of Heat Transfer Amritansh Manthapurwar V-A-09, MECHANICAL DEPT.
- 2. 2 Experimental investigation on natural circulation heat transfer of supercritical CO2 in a closed loop YuanluLi, Gongming Xin, Baoqiang Yuan, Shuangxing Zhang, Wenjing Du School of Energy and Power Engineering, Shandong University, Jinan 250061, China
- 3. 3 Highlights ● The flow and heat transfer of supercritical CO2 natural circulation in closed loop were studied. ● Normal heat transfer and heat transfer deterioration were also observed. ● A heat transfer deterioration criterion for natural circulation was obtained. ● A heat transfer correlation applicable to supercritical CO2 natural circulation is developed.
- 4. 4 ● The natural circulation system based on supercritical CO2 has attracted attention due to its simple structure and safety compared with the forced circulation system. This work presented an experimental investigation of the heat transfer characteristics of CO2 based on natural circulation. ● Two heat transfer modes can also be observed in supercritical CO2 natural circulation, normal heat transfer and heat transfer deterioration. The development of heat transfer deterioration criterion and heat transfer correlation is significant for the application of supercritical CO2 natural circulation, but there are few related studies. ● Direct adoption of the supercritical CO2 forced convective criteria and heat transfer correlations to natural circulation introduces large deviations. By analyzing the existing research results of forced convection in supercritical CO2 and introducing new dimensionless parameters SBO and K, a deterioration criterion and a heat transfer correlation suitable for natural circulation of supercritical CO2 are developed based on experimental data. ● The new criterion can well distinguish the two heat transfer modes, and the new correlation improves markedly. The results can benefit engineering applications in advanced power cycles. Abstract
- 5. 5 Discussion ● Supercritical fluids have been widely used in various industrial applications, such as chemical extraction, nuclear reactors, refrigeration systems, heat pumps and geothermal energy. Especially in the field of nuclear energy supercritical fluids are widely used and play an important role in the development of nuclear energy, and are considered to be the ideal working medium for GEN 4 nuclear reactors. Supercritical CO2 (sCO2) has received more and more attention due to its special thermodynamic properties. Carbon dioxide is inexpensive, environmentally friendly, abundant in reserves, and its critical pressure and temperature are 7.38 MPa and 31.1°C, respectively, making it easier to supercritical. ● The sCO2 has promising applications. However, in the pseudo-critical region, the physical properties of CO2 vary drastically with temperature, as shown in Fig. 1, resulting in complex heat transfer characteristics. Therefore, many scholars have conducted in-depth studies on sCO2 in recent years. However, due to the complexity of its heat transfer behavior, there are still doubts in some aspects.
- 6. 6 ● It is undeniable that there are two modes of heat transfer in the forced convection of sCO2 in a tube: normal heat transfer (NHT) and heat transfer deterioration (HTD). The normal heat transfer is consistent with the conventional single-phase heat transfer, where the wall temperature profile varies monotonically with the direction of flow, and we can use the Dittus-Boelter equation to predict the heat transfer. ● When the heat transfer deteriorates, a local high temperature will be generated in the heating section, and the temperature will increase abnormally and then fall back along the flow direction. This poses challenges to the safety of equipment. In order to deeply explore the application value of sCO2, many scholars were committed to the study of heat transfer deterioration prediction and heat transfer correlation of supercritical fluid.
- 7. 7 Fig. 1. Thermal-physical properties of supercritical CO2 at pressure of 7.5 MPa.
- 8. 8 ● Although research on supercritical fluid has been around since the 1930s, most of these studies were limited to the heat transfer characteristics of sCO2 under forced convection, while few studies on sCO2 natural circulation have been conducted. In a natural circulation loop (also called NCL or thermosyphon), the fluid is driven by buoyancy force resulting from the density difference between the hot and cold fluid. ● With the elimination of driving components, such as pumps, natural circulation systems are simple and reliable, thus outperforming forced circulation systems in terms of cost and system safety. Natural circulation loop based on supercritical fluid has been applied to nuclear cooling and in other energy systems, such as solar energy conversion, geothermal energy utilization. ● Liu et al. experimentally investigated the heat transfer characteristics of sCO2 in a rectangular circulation loop, in which significant heat transfer enhancement and heat transfer deterioration were observed. The effects of heat flux, pressure, buoyancy force on heat transfer of sCO2 were carefully discussed. They concluded that steep gradient of physical property over the tube cross-section is the key reason for heat transfer deterioration.
- 9. 9 ● Liu et al. then conducted a theoretical and experimental study of the steady-state characteristics of the sCO2 natural circulation. The effects of system pressure, inlet temperature and enthalpy difference on the steady-state characteristics were discussed in detail. And a new theoretical model was further validated. Deng et al. investigated the flow and heat transfer characteristics of the sCO2 natural circulation system both numerically and experimentally. The flow changes during the heat flux input variation were investigated, and it was found that the supercritical natural circulation flow is very sensitive to the heat flow variation. ● It is worth noting that prediction of heat transfer is important for sCO2 natural circulation applications due to the fact that heat transfer deterioration also occurs in natural circulation, but there is a lack of research on heat transfer prediction in sCO2 natural circulation. Fortunately, there is a lot of in-depth research on predicting heat transfer deterioration in forced convection, which provides a foundation for the study of heat transfer deterioration in natural circulation.
- 10. 10 ● As heat transfer deterioration can pose a challenge to the safety of equipment, many scholars have proposed various deterioration criteria based on forced convection of sCO2 in order to accurately predict the heat transfer deterioration. Table 1 summarizes several existing deterioration criteria. However, due to the complex heat transfer characteristics of sCO2, there is no unified conclusion on the heat transfer deterioration criterion. In the earliest study of forced convection of sCO2, it was found that heat transfer deterioration was related to heat flux qw and mass flux G, and a basic heat transfer deterioration criterion was proposed, which contained only qw and G parameters. ● The heat transfer deterioration and normal heat transfer can be distinguished by drawing qw∼G graphs, such as qw,CHF = 0.0002G2 proposed by Kim et al. [28], qw,CHF = 0.00316G1.49 proposed by Zhang et al. [29] and qw,CHF = 0.116G proposed by Shiralkar et al. [30], which all belong to this type of criterion. However, these criteria were found to be flawed in subsequent studies and only suitable for the scholar's own range of experimental parameters, which are difficult to extend.
- 11. 11 ● More results show that using two parameters, qw and G, to predict deterioration is not sufficient and that heat transfer deterioration is not only related to these two parameters. ● Therefore, more and more complex criteria based on qw and G were proposed, Cheng et al. took pressure P into account, Kline et al. took pipe diameter d into account, and Ma et al. criterion was more complex, and the fluid inlet temperature Tin, diameter d and pressure P were considered.
- 12. 12
- 13. 13 ● The development process through the deterioration criteria shows that new parameters are constantly introduced, which does not solve the problem fundamentally. In the recent years of sCO2 research, Zhu et al. provided a new theory. They analogized the heat transfer behavior under supercritical pressure with the growth and detachment of bubbles under subcritical pressure. Under the subcritical pressure two-phase flow, the force on the expansion of the bubble tends to attach the bubble to the wall, which is called the evaporation induced momentum force FM'v. On the other hand, the mass flux G exerts an inertial force FI’ on the bubbles, making the bubbles tend to detach the wall. ● The relative magnitude of the two forces is called the K number, Eq. (1), which reflects the competition between evaporation induced momentum force and inertia force. Where the term qw/(G·ifg) is the Bo (boiling number). A large K number indicates the vapor expansion to attach the bubble on the wall. Coalescence of various bubbles forms a vapor blanket to trigger the wall temperature overshoot. Alternatively, a small K number represents large inertia force to detach the bubble from the wall.
- 14. 14 ● Thus, the tube wall can be rewetted by liquid to keep better heat transfer performance. They compared supercritical heat transfer with boiling under subcritical pressure and proposed new dimensionless numbers supercritical K and SBO (supercritical boiling number). The basis of this theory lies in the fact that supercritical fluids are not homogeneous fluids, and their physical properties change drastically at a certain temperature and pressure, called the pseudo-critical point, which can divide supercritical fluid into two different regions, the liquid-like and gas-like regions, as shown in Fig. 2. K = (qw/G·ipc)2ρb/ρw, supercritical K reflects the competitive relationship between the two forces, momentum force and inertia force of gas-like layer evaporation, and reflects the influence of gas-like layer on heat transfer. ● Zhu et al. defined qw/G·ipc in K as SBO (supercritical boiling number), SBO = qw/G·ipc. The SBO was used to successfully predict the heat transfer deterioration of sCO2 forced convective. The proposed new parameters K and SBO provide a new method for supercritical fluid study.
- 15. 15 Fig. 2. Phase diagram of CO2.
- 16. 16 ● At the same time, fitting the heat transfer correlation is also critical to accurately predict Nu, which is essential for the application of sCO2 natural circulation. Due to the complexity of supercritical fluid heat transfer, it is difficult to obtain analytical solutions, most of which are empirical correlations. Table 2 lists four typical heat transfer correlations with different structures and different ranges of applicable parameters. Jackson et al. and Gupta et al. correlations are two widely used sCO2 forced convective heat transfer correlations. Liu et al. proposed a heat transfer correlation in the study of sCO2 natural circulation, and the buoyancy Bu was introduced into the correlation. ● However, due to the lack of relevant research on the development of sCO2 natural circulation heat transfer correlation, there is no widely accepted heat transfer correlation. Locally, the sCO2 flow is caused by the pressure difference between the inlet and outlet, which is consistent with the forced convection. However, on the whole, the driving force of the system is caused by the change of the physical properties of sCO2, which is different from the forced convection.
- 17. ● The flow and heat transfer behaviors of natural circulation require further discussion compared to forced convection. The applicability of forced convection correlations is also discussed further, and a correlation suitable for natural circulation needs to be developed on the basis of forced convection. 17 ● In this study, an experimental system for natural circulation of CO2 in a closed loop was established. The general trends and heat transfer characteristics of natural circulation were analyzed. ● The effects of operating parameters on flow and heat transfer were discussed in detail, such as heat flux, mass flux, pressure and filling mass. Subsequently, the sCO2 forced convection deterioration criteria and correlations were analyzed and compared, and a deterioration criterion and heat transfer correlation with higher prediction accuracy were developed for the natural circulation of sCO2 based on experimental data. The results of the study can provide theoretical guidance for the design and optimization of the sCO2 power cycle.
- 18. 18
- 19. CONCLUSION The experiment investigates the flow and heat transfer characteristics of supercritical CO2 in a closed natural circulation loop. The following conclusions are obtained: 1. A closed loop natural circulation system is built. The general trend and heat transfer characteristics of sCO2 natural circulation are analyzed. There are also two heat transfer modes in the natural circulation, normal heat transfer (NHT) and heat transfer deterioration (HTD). 2. The transition of heat transfer mode is analyzed and the existing HTD criteria of sCO2 are evaluated. A new dimensionless parameter, SBO, is introduced to develop a heat transfer deterioration criterion for the natural circulation of sCO2, qw,CHF = 3.05 × 10−4G·ipc. 3. The existing sCO2 forced convection heat transfer correlation is not applicable to natural circulation. By comparing and analyzing several typical sCO2 heat transfer correlations and introducing the dimensionless parameter K number into the sCO2 natural circulation heat transfer correlation, a heat transfer correlation applicable to sCO2 natural circulation is developed. The new correlation improves markedly, which captures 96.31% of the data with an accuracy of ±30%. 19
- 20. References ● https://www.sciencedirect.com/science/article/pii/S0017931022009723 20
- 21. THANK YOU 21