Study on the Mitigation of Heat Transfer Deterioration of Fluids at Supercritical Pressures


Student thesis: Doctoral Thesis

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Award date3 Sep 2020


The occurrence of heat transfer deterioration (HTD) in supercritical fluid systems, where the wall temperature rises abruptly is an undesirable phenomenon that is limiting the design of new promising engineering applications. This may cause operational problems such as tube burnouts which could result in a catastrophic system failure. In this thesis, numerical studies are conducted to determine the performance effects of flow obstacles on HTD mitigation of supercritical fluids flowing vertically and horizontally in macro and mini channels using water and CO2 as the working fluids. The results presented here can be employed as a reference guide in the design of highly efficient, safe, and reliable supercritical heat exchanger systems. 

For the macro channels, vortex generators (VGs) are installed as protrusions to the flow channels, and numerical investigations are carried out for supercritical water flowing vertically at high heat flux and low mass flux using SST k-ω turbulence model. The results indicate that VG’s size slightly suppresses and delays HTD downstream of the VG, and there exists an optimum size beyond which HTD starts to aggravate. A strong effect of VGs on heat transfer coefficient (HTC) profiles is observed at locations where VGs are installed and at the downstream locations, with the normalized HTC showing that the wall temperature fluctuates a couple of times before becoming stable once the flow returns to fully developed state. The VG’s position has the most significant effect on HTD for any single VG. Increasing the number of VGs installed in-line at the start of the major peak baselines significantly suppresses and delays the peak. Mechanism analysis based on radial distributions of velocity and turbulent kinetic energy (TKE) at different axial positions of both channels shows that HTD suppression is caused by the weakening of the buoyancy effect due to increased TKE near the wall downstream of the VG, whereas the delay is caused by the boundary layer recovery effect due to flow redevelopment after being disrupted by the VG.

In addition, a novel small-scale multiple vortex generators (MVG) is introduced and its performance effects on HTD mitigation and pressure drop using supercritical CO2 (sCO2) as the working fluid are explored based on the overall performance factor, R = ΔHTD/(Δp/Δp0). Numerical calculations are first conducted to investigate the effects of single conventional VG configurations (rectangular, triangular, and trapezoidal). The results indicate that under the same aspect ratio, rectangular VG has the highest value of R. Second, the morphological properties (density and height) are varied while their uniformity is maintained. With the increase in MVG’s height, the channel’s HTC profiles increase significantly at the locations where MVG is installed while the opposite trend is found for R. The density and non-uniformity of the MVG have the highest performance effects on HTD mitigation, with R of more than 18% and 11% for the density of 2.5 and non-uniform MVG respectively. Analysis reveals that as the density of MVG increases, the fluid recirculation zone behind each VG diminishes correspondingly, causing a greater vortex intensity mixing by swirling flows which enhances the downstream production of TKE, thus weakening the buoyancy forces and leading to HTD mitigations. In addition, by further increasing the MVG density beyond 2.5, it was found that the HTC decreases while the pressure drop increases, with the wall temperature peaks stabilizing at 178°C, which demonstrates the breakdown of Reynold’s analogy. 

Responding to the rapid growth of size reduction in engineering systems, it is crucial to explore the safety performance of supercritical fluid systems in miniature tubes. To this end, numerical studies are conducted to explore the performance of baffles on heat transfer deterioration (HTD) mitigations for supercritical CO2 (sCO2) flowing in a horizontal mini tube using RNG k-e turbulence model with enhanced wall treatment (EWT). Three kinds of baffle arrangements: in-lined, staggered and centred, with the same blockage ratio (BR = 0.25) are selected to explore their performance on HTD mitigation. The thermo-hydraulic performances of the baffled tubes are examined using the dimensionless parameters based on normalized Nusselt numbers and Fanning friction factors, Nu/Nu0, f/f0 and PEC = (Nu/Nu0)/(f/f0)1/3. The results reveal that unlike the in-lined and staggered baffles, the centred baffles induce jet impingements directly onto the tube walls and generate significant amounts of transverse fluid velocity near the tube wall, making it the most influential baffle arrangement on the HTD mitigations. The effects of baffles on buoyancy flow under a wide range of heat and mass fluxes are explored based on Jackson’s criterion (Gr/Re2<103). It is observed that the Gr/Re2 increases at a relatively lower rate in baffled tube than in the smooth tube under increased heat flux variation whereas it decreases at a higher rate than the smooth tube under increased mass flux variation, thus revealing the buoyancy weakening effects of baffles. Finally, the baffle’s performance on HTD mitigation when the tube is further miniaturized is explored and the results unveil that miniaturization weakens buoyancy, and there exists a certain mini size (D = 0.5 mm) at which buoyancy influence completely vanishes, and the effect of baffles on HTD mitigation becomes insignificant.