SINGLE- AND MULTI- PHASE FLUID FLOW AND HEAT TRANSFER IN CURVED GEOMETRIES

Abstract

Curved geometries can improve the heat transfer performance of a heat exchanger by virtue of curvature, which alters the fluid flow and heat transfer characteristics. Curved configurations can allow for a more compact heat exchanger design, which can be useful in situations where space is limited. Healthcare equipment, biomedical engineering, waste-water treatment, heat transfer devices, and other fields use curved geometries extensively. Despite the widespread use of curved geometries in process industries, performance evaluation is still unclear, and there is still essentially no fundamental basis for a quantitative and theoretical understanding. This study investigates the single- and multi- phase fluid flow and heat transfer characteristics in curved geometries to achieve the most efficient heating and cooling by means of incremental design advantage of a curved surface embedded with fluid dynamics. The essential literature for the current study is presented, which also includes a review of numerical and experimental methods for single- and multi- phase heat transfer with gaps in literature, the driving force behind the study, and its goal. Firstly, curvature features of heat transfer and fluid flow for both Newtonian and non-Newtonian fluids in tube-in-tube helical coil (TTHC) heat exchangers have been investigated, considering temperature-dependent thermophysical properties. The k- turbulent model is used to simulate turbulent flow in tube-in-tube helically coiled heat exchangers. The performance of TTHC heat exchanger was studied for four different configurations: 1) parallel flow with baffles, 2) parallel flow without baffles, 3) counter flow with baffles, and 4) counter flow without baffles. The power law index (n) and Dean Number (De) are varied from 50 to 500 and 0.5 to 1.25, respectively. It has been found that friction factor (f) and Nusselt number (Nu) in the TTHC heat exchanger with annular baffles are higher than TTHC heat exchanger without baffles. Additionally, the baffles have a substantial impact on heat transfer at low Prandtl numbers, whereas flow configuration is highly relevant at high Prandtl numbers. Two distinct models for the prediction of friction factor and Nusselt number with different configuration have been developed, i.e., generalized and power law models. The results from the generalized model for the Nusselt number were found to be very close to the present numerical prediction. Effectiveness-NTU analyses were in very good agreement with the analytical predictions. In the second part of the present study, boiling is taken into account in multi-phase heat transfer phenomena over curved surfaces. Boiling got key interest in recent decades due to its application to compact heat exchangers, steam tubes in boilers, tubes in refrigerant industries, compact evaporators, and microelectronics devices with heat pipes, etc. Boiling events cause a vapour to coat the heater's surface, reducing the total heat transfer coefficient. By allowing the vapour to slide over the curved surface, the surface's curvature aids in reducing the amount of vapour blanketing. The sliding creates fluid circulation and opens up the heater's surface so that liquid can boil. In the present study, the effect of curvature, degree of superheat and orientation on the vapour film dynamics over different curved surfaces (2-D circular and elliptical, and 3-D helical coils) have been studied. Volume of fluid (VOF) with a suppressed interface tracking and phase change methods have been used to forecast vapour dynamics over curved geometries for the multi-phase heat transfer problems. For the mass transfer from one phase to another, the Tanasawa model was employed. The features of heat transport, bubble pinch-off, and nucleation and growth over smooth, uniform curved surfaces with varying curvature ratios ( = 0, 0.01, 0.05, 0.15, and 0.25) are numerically investigated. The bubbles formation, vapour sliding, and fluid recirculation along the curved surface have been observed as a function of the degree of superheat (T = 10, 20, 30, and 40K). Three different departure patterns were observed, which are further categorized into three major types: (1) symmetric, (2) dome, and (3) multiple departures. Boiling over a curved surface causes the fluid to slide and circulate, which improves surface renewal and raises the heat transfer coefficient. It was found that the curved surfaces have an 86% increment in average HTC as compared to the flat surface. The above study has been further extended to investigate the boiling phenomena over spherical surface at various degrees of superheat (T), ranging from 10K to 500K. For a thorough knowledge of the boiling process over a spherical surface, several phenomena including vapour sliding, bubble formation, pinch-off, and induced vorticity have been demonstrated. The FFT of a spaced average liquid void fraction and Nusselt number revealed that film boiling predominated with respect to the level of superheat. Correlations for vapour volume generation with time and degree of superheat have been developed using curve fitting as well as ANN model. In addition, to examine the effect of surface curvature on saturated pool boiling in atmospheric circumstances, two elliptical surfaces (E1 and E2) with three different orientations (0°, 45°, and 90°) have been taken into consideration. The boiling heat transfer analysis is carried out across a broad temperature range (50 K to 500K). The difference in the sliding of vapour differs the bubble pinch-off volume and time. E1 and E2 generated 60.31% and 64.19%, respectively, more vapour volume as compared to circular surface (t = 0.24) at a degree of superheat (T) of 50K. As per orientation is concerned E1 with 0 angle produced 110% more vapour than E1 with 90 angle. Heat transfer parameters have also been studied in terms of Nusselt number and contrasted with the circular surface, along with fluid mechanics. Further, the physics of two-phase boiling of water has been numerically studied over the heated surface of a helically coiled tube at a critical pressure (Psat = 21.9MPa). Typically, such high-pressure systems are found in boiling water reactors (BWR). The dynamics of the vapour bubbles, including bubble merging and separation, have been investigated for eight different helically coiled heated surfaces with varied curvature ratios () from 0-7.16 and coiled tube orientations (vertical and horizontal) at a degree of superheat (T) of 10K. The orthogonality of the surface normal and gravitation force make vapour move faster. Thus, H1 (horizontally oriented tube) configuration possesses a 3.79 times higher average velocity than H4 (coiled tube) configuration. Vertical configuration, i.e. helically coiled tube (V4) with curvature ratio ( = 0) produced 4.4 times higher vapour than that produced by horizontally oriented helically coiled (H4) configuration as it has more surface renewal characteristics (from FFT analysis) as compared to other considered cases. It was concluded that fluid dynamics, heat transport, and the creation of vapour volume, during boiling heat transfer, all depend significantly on surface curvature and orientation.