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Thermal and Mechanical Design
Published in Shen-En Qian, Hyperspectral Satellites and System Design, 2020
PTCs are similar to Stirling cryocoolers except that a pulse tube replaces the moving displacer. The thermodynamic processes of the PTCs are somewhat different from those of Stirling cryocoolers. A PTC is mechanically simpler than Stirling cooler because it has no moving parts at the cold end. This can increase cooler reliability of a system and make integration with the payload much simpler if the thermal margins are comparable. Stirling cryocoolers have fundamentally higher efficiency where optimized, requiring lower input power to achieve the same cold temperature, which is emphasized at lower cold-tip temperatures. Stirling cryocoolers also offers some functionality to shift heat loads and adjust performance by controlling the phase of the displacer, which could be advantageous if ever a leak affected the resonance of the unit. Stirling cryocoolers can use the displacer (cold-head mechanism of a Stirling) and can help to achieve lower vibration at the cold-tip itself, where this has been problematic in the case of some PTC designs. One cannot assume that the vibration at a pulse tube cold-tip is null. The compressors are balanced, but the gas slug moving the cold-head can be a significant source of micro-vibration.
An investigation on the effect of divergent angle on performance of Venturi-type bubble generators
Published in Heping Xie, Jian Zhao, Pathegama Gamage Ranjith, Deep Rock Mechanics: From Research to Engineering, 2018
Liang Zhao, Licheng Sun, Zhengyu Mo, Min Du, Guo Xie, Jiguo Tang
In the diverging section of the Venturi channel, interesting phenomena of gas-liquid two-phase flow is involved due to the enhanced turbulent flow inducing strong interactions between the liquid and gas. Fig. 4 presents typical gas transportation and breakup processes in the diverging sections with different divergent angles at different liquid and gas flow rates. As gas bubbles or slugs entered the diverging sections, they were decelerated and hindered to move forward by the recirculation flow near the wall. Consequently, the interactions between gas and liquid were intensified and the gas bubbles or slugs were collapsed into lots of tiny bubbles. The produced bubbles under different conditions of liquid flow rates and divergent angles are shown in Fig. 5. It was clearly shown that the larger liquid flow rate and divergent angle, the stronger interaction between gas and liquid. It was resulted from the combined interaction between the recirculation flow and the gas slug moving forward. The liquid recirculation flow hindered the gas slug from moving forward, while the eddy impacted the gas slug and collapsed it into a quantity of fine bubbles subsequently. An increase in the divergent angle led to the impact position moving forward and created more intensified collapse of bubbles. This proved the key role of the diverging section in triggering bubble breakup.
Gas–Liquid Flow in Ducts
Published in Efstathios E. Michaelides, Clayton T. Crowe, John D. Schwarzkopf, Multiphase Flow Handbook, 2016
Afshin J. Ghajar, Swanand M. Bhagwat
It is seen that for a xed liquid ow rate, when the gas ow rate is increased, the two-phase pressure gradient exhibits rst a minimum at FrSG 0.2 - 0.3 and a maximum at FrSG 0.4. e rst instance of decreasing pressure drop is due to falling lm surrounding the gas slug. As the gas slug rises in downstream direction it sheds liquid phase surrounding it to maintain the continuity. e sudden increase in pressure drop between FrSG 0.2 - 0.4 is due to the high level of turbulence caused by disintegration of gas slug during slug to churn/ intermittent ow transition. Further this point of maximum pressure gradient at FrSG 0.4, churn/intermittent ow is known to commence and again a decreasing trend of pressure gradient followed by a pressure gradient minimum is observed at FrSG 0.9 - 1. Beyond this point, the annular ow is known to exist. Note that this value of the nondimensional gas ow rate is essentially the criteria given by Equation 3.26. e second trend of decreasing pressure drop could be explained using Figure 3.26. During churn annular ow transition, large interfacial disturbance waves are generated that travel in downstream direction (Figure 3.26-sketch b). During the swiping action of disturbance wave, the liquid lm travels in downstream direction under the in uence of interfacial drag. However, once the disturbance wave passes by a certain pipe cross section, there is no driving potential for the liquid lm and it tends to fall back under the in uence
Gas-Liquid Two-Phase Flow Regime in a Horizontal Channel Under Transverse Vibration
Published in Nuclear Science and Engineering, 2022
As shown in Fig. 3b, no matter under steady state or transverse vibration, the gas phase was attached to the upside wall of the channel in the form of narrow bubbles. Gas slug adhered to the upside wall in a constant shape with the flow of the liquid phase. Under the effect of transverse vibration, the interface of the gas slug fluctuated randomly. Irregular changes in bubble shapes began to appear. It can be concluded that, with the increase of gas velocity or vibration amplitude, narrow bubbles would adhere to each other, thus forming a continuous interface. Besides that, it can be seen from Fig. 3b that a mass of tiny bubbles stuck to the gas column. As the flow continued, most of them were suspended in the upside wall of the channel, and other tiny bubbles gathered in the tail part of each gas slug. This phenomenon can be explained as follows.
Review on Heat and Fluid Flow in Micro Pin Fin Heat Sinks under Single-phase and Two-phase Flow Conditions
Published in Nanoscale and Microscale Thermophysical Engineering, 2018
Koşar [99] further analyzed the configuration in Koşar et al. [100]. For all mass fluxes, pressure drop slightly decreased with heat flux in the single-phase regime. This trend continued until the minimum at the onset of significant void (OSV). Beyond this point, pressure drop increased until the critical heat flux (CHF) condition. A delay in OSV was reported with increasing mass flux. The two-phase pressure drop patterns were classified into three groups, namely bubbly, wavy-intermittent and spray-annular patterns, using the Lockhart-Martinelli model [98] with the recommended coefficients. The predictions were accurate for laminar liquid and gas flows. However, the data for other liquid and gas flows were overpredicted. The model by Schrage et al. [97] predicted the data well for spray annular flow, while the discrepancies became larger for other flow patterns as a result of more vigorous separation of liquid and gas regions in the spray annular flow. He suggested that the Lockhart-Martinelli parameter (Xvv) plays an important role in characterization of flow patterns with critical values of 1.8 and 3. These values marked transitions from bubbly to wavy intermittent and from wavy intermittent to spray annular, respectively. Krishnamurthy and Peles [101] considered the same configuration as in their previous study [96] using ethanol. They observed the same two-phase flow patterns as with water. However, the transition boundaries were different. The transitions from bubbly to gas slug flow and from gas-slug to bridge pattern were delayed to higher gas flow rates. A reverse trend was seen for transition from bridge to annular flow. They suggested that the void fraction was a weak function of surface tension and depended more strongly on liquid density and viscosity. As pressure drop dependency on gas flow Reynolds number was different for ethanol, lower two-phase pressure drops were obtained compared to water. A new correlation for interfacial friction factor based on Rahman et al. [102] and a modified two-phase friction multiplier based on their previous study [96] were proposed (Table 7). As shown in Figure 16, the characteristic length of bubbles became smaller than the pitch distances in case of ethanol.