Small Flow Rate Research Articles

Comparison of Saddle-Shaped Region of Head-Flow Curve between Axial-Flow Pump and Its Corresponding Axial-Flow Pump Device

In engineering, the highest operating head of the pumping station is usually controlled to be slightly lower than the lowest saddle bottom head of the axial-flow pump. However, in the practical operation, it is found that the highest operating head of the pumping station is obviously lower than the saddle bottom head of the pump device, which leads to the reduction of the operating range of the pumping station. To investigate the difference of lowest saddle bottom head between axial flow pump and axial flow pump device and apply it correctly, the energy performance tests of the TJ04-ZL-06 hydraulic model and its corresponding pump device were carried out to obtain the external curves, and numerical simulation was carried out to analyze and compare the internal flow field and pressure distribution. The results show that when the flow rate decreases, the first saddle-shaped region of the axial-flow pump and the saddle-shaped region of the pump device are caused by the decrease of the lift coefficient due to the increase of the attack angle between flow and blade. When the flow rate is less than 0.32Qd, the influence range of backflow in the inlet pipe is large, which leads to the high-pressure zone near the wall of the inlet pressure measurement section during the pump performance test, and hence the second saddle-shaped region of the axial-flow pump is essentially a measurement illusion. It is suggested that the inlet pressure measurement section should be set at least 4Dp away from the inlet flange of the impeller when testing the performance of the axial-flow pump under the condition of small flow rate, and the first saddle bottom head of the axial-flow pump or the saddle bottom head of the corresponding pump device can be considered as the control value of the highest head of the pumping station.

Performance measurement and evaluation of an ionic liquid electrospray thruster

As a novel micro-propulsion system for small satellites (from micro to nano), the ionic liquid electrospray propulsion system is a promising candidate. However, performance measurement and evaluation of the Ionic Liquid Electrospray Thruster (ILET) is one of the most challenging issues for practical application, due to the difficulties in the development of a prototype and direct measurements of micro-thrust and small flow rate. To address this issue, a Modular Ionic Liquid Electrospray Thruster (MILET) prototype is constructed, and a diagnostic system for thrust and mass flow rate is specially developed based on an analytical balance method. With the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate as the propellant, a series of experiments is carried out on the MILET prototype under a wide operating condition through changing the applied voltage to control the thrust. Under different applied voltages, the thrust and the mass flow rate of the propellant are directly measured. The propulsive performance parameters of the thruster, such as thrust, specific impulse, thrust-to-power ratio, thruster efficiency, etc., are comprehensively analyzed. Then, a performance comparison is made between the MILET and other representative ILETs. With a relatively low applied voltage ranging from 1550 V to 2000 V, the MILET achieves a quasi-constant specific impulse of 1263 s with the averaged thrust-to-power ratio of 65.2 μN/W and thruster efficiency of 40.7%. The performance of ILET is also compared with other typical electric propulsions. The results demonstrate that the ILET exhibits an excellent ability of minimalization with high specific impulse and thruster efficiency, which guarantees a great superiority in micro propulsions. Finally, the ways to further improve the performance of ILET are discussed, which further confirms the potential prospect of ILET. The present result helps to advance the development and application of ILET.

Comparison between the cooling performances of micro-jet impingement systems using liquid metal and water as coolants for high power electronics

Liquid metals are attractive coolants for MEMS with a high heat flux. However, most existing studies on liquid metals were focused on micro- and mini-channel heat sinks. In the present work, we applied liquid metals into three micro-jet impingement systems, i.e., jet impingement surface cooling (JISC), jet impingement body cooling (JIBC) and hybrid body cooling (HBC) systems. Analytic and numerical models were established, based on which the thermal resistances of these systems using water and liquid gallium (Ga) as coolants were compared and analyzed. The results indicated that the systems using liquid Ga always achieved a smaller thermal resistance compared to water, showing a maximum decrease of 29.8% in thermal resistance and a largest drop of 12.6 K in chip temperature. The minimum thermal resistance using liquid metals reaches as low as 0.033 K/W. In addition, JIBC system always had the smallest thermal resistance when water was used. However, in the case of liquid Ga, HBC system performs best at small flow rates. The influences of geometry parameters on the system cooling performance were analyzed. It was shown that an optimal nozzle spacing of 4.7 mm existed. Moreover, the thermal resistances of the three systems increased with an increasing nozzle diameter, and the thermal resistance of JIBC system would exceed that of HBC system at an enough large nozzle diameter (e.g., ∼0.28 mm for Ga). Regardless of the coolant used, the thermal resistances of JIBC system kept nearly constant with the variation of side gap height. For HBC system, the thermal resistance increased obviously with an increasing side gap height when water was used as coolant, while barely changed for liquid Ga. The findings in this study provide a guidance for the design and operation of micro-jet impingement systems employing liquid metals as coolant.

Experimental study of transition to instability in a Rijke tube with axially distributed heat source

Self-excited thermoacoustic oscillations are favorable in thermoacoustic engines, but unwanted in many combustion systems due to the detrimental outcomes, including the structure vibration and overloaded thermal flux to combustor wall. To explore some proper methods to modify thermoacoustic oscillations, understanding the mechanism of thermoacoustic instability is needed. In the present work, the transition to instability in a Rijke-type thermoacoustic system with axially distributed heat source is explored experimentally. A silicon/ceramic tube is used as the acoustic resonator, and the distributed heat source is designed by wounding electric wires over two ceramic rings. The influences of the characteristics of heat source (including heating power, heater length and heater location), mass flow rate, and tube materials on the nonlinear dynamic behaviors and stability boundaries of the Rijke-type thermoacoustic system are systematically evaluated. Large-amplitude limit cycle is observed, and subcritical and supercritical bifurcations are present in the thermoacoustic system. For the system with fixed mass flow rates, the critical heat power for transition to instability is found to be approximately linearly proportional to the heater length, while the strength of pressure oscillations responds nonlinearly. In addition, the system with a thin heater is more prone to undergo subcritical bifurcation. As for the influence of the mass flow rate, it has also been found that increasing the mass flow rate would enhance the nonlinearity of the coupling between the unsteady heat release rate and acoustic waves, which leads to a high possibility of occurrence of subcritical bifurcation. For all the heater length studied, there is an optimal mass flow rate where the critical heat power for the transition to instability is minimal. Last, the results show that the thermoacoustic system with ceramic tube has a narrower stability region at small air flow rate and is more prone to subcritical bifurcation than the silicon tube.

Second law performance of a novel four-layer microchannel heat exchanger operating with nanofluid through a two-phase simulation

A 3D two-phase analysis is conducted to evaluate the second law features of a counter-flow four-layer microchannel heat exchanger (MCHE) operating with an Al2O3–water nanofluid. The coolant is considered water, while the hot fluid is the nanofluid. Three MCHEs with varied number of channels are examined. This investigation highlights the minimization of the exergy destruction to figure out the conditions in which the amount of degraded energy is lowest. Suspending more nanoparticles diminishes the thermal irreversibility of the nanofluid up to 61.6%, whereas the cold water experiences around 11.7% higher thermal irreversibility. At the small flow rates, increasing the volume fraction causes smaller exergy destruction, while at great flow rates, the trend becomes slightly ascending. The greater flow rate results in the greater thermal and frictional entropy generations. The second law efficiency firstly escalates via increasing the volume fraction but then remains almost constant or even descends somewhat.

Instability of a liquid sheet with viscosity contrast in inertial microfluidics

Flows at moderate Reynolds numbers in inertial microfluidics enable high throughput and inertial focusing of particles and cells with relevance in biomedical applications. In the present work, we consider a viscosity-stratified three-layer flow in the inertial regime. We investigate the interfacial instability of a liquid sheet surrounded by a density-matched but more viscous fluid in a channel flow. We use linear stability analysis based on the Orr–Sommerfeld equation and direct numerical simulations with the lattice Boltzmann method (LBM) to perform an extensive parameter study. Our aim is to contribute to a controlled droplet production in inertial microfluidics. In the first part, on the linear stability analysis we show that the growth rate of the fastest growing mode xi ^{*} increases with the Reynolds number text {Re} and that its wavelength lambda ^{*} is always smaller than the channel width w for sufficiently small interfacial tension Gamma . For thin sheets we find the scaling relation xi ^{*} propto mt^{2.5}_{s}, where m is viscosity ratio and t_{s} the sheet thickness. In contrast, for thicker sheets xi ^{*} decreases with increasing t_s or m due to the nearby channel walls. Examining the eigenvalue spectra, we identify Yih modes at the interface. In the second part on the LBM simulations, the thin liquid sheet develops two distinct dynamic states: waves traveling along the interface and breakup into droplets with bullet shape. For smaller flow rates and larger sheet thicknesses, we also observe ligament formation and the sheet eventually evolves irregularly. Our work gives some indication how droplet formation can be controlled with a suitable parameter set {lambda ,t_{s},m,Gamma ,text {Re}}.Graphical

Thermal Analysis of a Forced Flow Diffusion Absorption Refrigeration System for Fishing-Boat Exhaust Waste Heat Utilization

Ammonia water absorption refrigeration systems are effective in utilizing fishing-boat exhaust waste heat for cryopreservation. However, the liquid level control and the use of a solution pump characterized by small flowrate and high-pressure head result in poor reliability in the traditional system. Besides, the system must necessarily be designed anti-swaying and anti-corrosion. This paper proposes a forced flow diffusion absorption refrigeration system, in which an inherently leak-free canned motor pump and an ejector are employed to provide the driving forces of the gas and liquid loops. The approximate single pressure operation allows for a simple passive liquid sealing control without throttling valves. The system adopts an integrated cooling strategy which allows the system to operate under swaying conditions, and the external seawater cooled heat exchanger avoids internal corrosion and leakage. The thermal analysis shows the system is valid to be operated under wide operating conditions, and the coupled gas and solution circulation ratios determined the performance of the novel system. There is an optimal ammonia mass fraction difference in the gas loop to obtain the optimal COP. The COP reaches 0.4 when the temperatures at the outlets of the generator, evaporator, absorber, and condenser are 160, −15, 35, and 35°C, respectively. The novel system provides a reliable absorption refrigeration system design for fishing-boat applications.

A new benchmark for the secondary fluid flow through curved ducts

Steady flow through curved ducts has numerous applications in the chemical engineering sciences. A recent example is within the field of microfluidics in which curved ducts have cross-section dimensions much smaller than one millimetre. The validation of steady fluid flow through curved microfluidic ducts is a crucial part of verifying any experimental work or numerical modelling in this field. Of particular interest is a measure of the secondary flow which consists of a counter-rotating vortex pair in the cross-sectional plane. We develop a new model for the magnitude of the secondary flow velocity within curved ducts having rectangular cross-sections. Our modelling respects the scaling at asymptotically small flow rates while carefully considering how this is modified for increasing Dean number. Lastly, we discuss why this new model is a more appropriate benchmark of the secondary flow than a power law model commonly found throughout the literature.

Aerodynamic design and experimental validation of high pressure ratio partial admission axial impulse turbines for unmanned underwater vehicles

High pressure ratio partial admission axial impulse turbines are typically employed for underwater vehicles due to small mass flow rate and machining constraints. However, little work has been focused on the generic design methodology for high pressure ratio partial admission axial impulse turbines. The experimental results and empirical correlations are mostly established upon low pressure ratio axial turbines. In this paper, the meanline turbine design method is first documented together with the performance prediction method at off-design points. The numerical loss breakdown method is then described to thoroughly verify empirical correlations. Both the meanline and numerical methods are thoroughly validated against experimental results and the difference among meanline, numerical and experimental results is less than 10%. A case study is then performed and a 13 kW partial admission axial impulse turbine is designed. The suitability of empirical correlations is confirmed by comparing the losses between the meanline and numerical methods, and the difference less than 5% at design and off-design points is attained. This paper provides the insight into the design methodology for high pressure ratio partial admission axial impulse turbines.

Differences in Formation Mechanism of Static Pressure Circumferential Distribution of Centrifugal Compressor Under Different Operating Conditions

Abstract The casing-wall static pressure of the centrifugal compressor behaves the double-peak distribution in the circumference at small flow rates (SM) but the single-peak distribution at large flow rates (LM). A previous study shows that the double-peak distribution is induced by the redistribution of impeller outlet flow rates. In this paper, by using the similar simplified method of directly imposing pressure boundary to the diffuser outlet, the original reason for the formation process difference of pressure distribution in the circumference at different operating conditions is further investigated. The results show that at LM, under the combined action of the specific downstream pressure distribution and the flow performance of the compressor itself, alternating low/high velocity airflow zones similar to those at SM cannot be established in the diffuser when the impeller outlet flow rates are redistributed. Therefore, the static pressure can only express the single-peak distribution in the circumference. In fact, whether the static pressure exhibits the double-peak or single-peak distribution in the circumference depends on whether the impeller outlet flow mutation can destroy the original flow balance. When the flow mutation is dominant, the double-peak distribution is created, whereas when the original flow balance is prevailing, the single-peak distribution is formed.

Applicability study of micro Kalina cycle for regional low grade geothermal heat in Japan

In this study, the micro Kalina cycle (Micro KC) was designed as 10 kW class power generation system to utilize the low grade thermal heat sources (hot spring) at a temperature of 373 K with applying characteristic of the Lorenz cycle. The utilization of low grade geothermal energy which is not currently the focus of attention, is important as an energy source for distributed energy systems in small villages in mountainous areas of Japan. In the case of low grade geothermal heat utilization, it is difficult to utilize for power generation due to its large temperature gradient caused by the low temperature and small flow rate of the heat source. This condition is suitable for applying the Lorenz cycle. It was found that micro KC could generate 9.42–11.0 kW of electricity under realistic hot spring water conditions (373 K, 4000 kg/h) of Japan. The micro KC showed in 50% higher power performance than a micro ORC using R245fa.

Canopy-to-canopy liquid cooling for the thermal management of lithium-ion batteries, a constructal approach

With the growing interest on electric vehicles comes the question of the thermal management of their battery pack. In this work, we propose a thermally efficient solution consisting in inserting between the cells a liquid cooling system based on constructal canopy-to-canopy architectures. In such systems, the cooling fluid is driven from a trunk channel to perpendicular branches that make the tree canopy. An opposite tree collects the liquid in such a way that the two trees match canopy-to-canopy.The configuration of the cooling solution is predicted following the constructal methodology, leading to the choice of the hydraulic diameter ratios. We show that such configurations allow extracting most of the non-uniformly generated heat by the battery cell during the discharging phase, while using a small mass flow rate.

Prediction of atherosclerotic changes in cavernous carotid aneurysms based on computational fluid dynamics analysis: a proof-of-concept study.

Recent computational fluid dynamics (CFD) studies have demonstrated the concurrence of atherosclerotic changes in regions exposed to prolonged blood residence. In this proof-of-concept study, we investigated a small but homogeneous cohort of large, cavernous carotid aneurysms (CCAs) to establish the clinical feasibility of CFD analysis in treatment planning, based on the association between pathophysiology and hemodynamics. This study included 15 patients with individual large CCAs. We identified calcifications, which indicated atherosclerotic changes, using the masking data of digital subtraction angiography. We conducted a CFD simulation under patient-specific inlet flow rates measured using magnetic resonance (MR) velocimetry. In the post-CFD analysis, we calculated the blood residence time ([Formula: see text]) and segmented the surface exposed to blood residence time over 1s ([Formula: see text]). We measured the decrease in volume after flow diversion using the original time-of-flight MR angiography data. Calcifications were observed in the region with [Formula: see text]. In addition, the ratio of [Formula: see text] to the surface of the aneurysmal domain exhibited a negative relationship with the rate of volume reduction at the 6- and 12-month follow-ups. Post-CFD visualization demonstrated that intra-aneurysmal swirling flow prolonged blood residence time under the condition of a small inlet flow rate, when compared to the aneurysmal volume. The results of this study suggest the usefulness of CFD analysis for the diagnosis of atherosclerotic changes in large CCAs that may affect the therapeutic response after flow diversion.

Thermal performance enhancement and prediction of narrow liquid cooling channel for battery thermal management

A novel heat transfer device for narrow space was proposed to meet high power battery heat dissipation. Based on the battery heat generation data, the heat transfer enhancement mechanism of the secondary flow pattern is studied. Then establish the general heat transfer correlation for this liquid-cooled structure with a specific aspect ratio. The results show that the channel with rib angle of 90° has higher Nu as well as ƒ and can achieve the same heat transfer performance with smaller flow rate. This is due to its transition flow pattern of twist-tape vortex and stagnant vortex, which can enhance the contact heat transfer performance of mainstream cold fluid while inhibiting the growth of boundary layer. The increase in rib number gradually decreases the gain of Nu with a minimum increment of less than 1%; while ƒ almost gradually increases with a common difference of 0.05. From the perspective of heat transfer and energy saving, increasing rib number does not essentially improve the heat transfer performance. Because the flow field at different rib number is just a mapping of the complete flow field at the corresponding length. Based on the above data, for the flat channel with aspect ratio of 10:3 in narrow space, the second-order general heat transfer correlation for different hydraulic diameter is established considering the Re, dynamic viscosity, rib angle and rib number. Finally, the coefficient of determination (R2) of the correlation is 0.9973, which can provide guidance for battery thermal management engineering applications.

Numerical investigation on novel water distribution for natural draft wet cooling tower

Natural draft wet cooling tower is widely used in cold end system of thermal power plants, however, the water droplets in the central area cannot be sufficiently cooled due to small air flow rate. In this paper, a novel water distribution mode is proposed for the wet cooling tower, in which the water distribution area is divided into four subdomains equally in the radius direction, with the diameters of sprayed water droplets from the inside to the outside of 4 mm, 3 mm, 1 mm and 0.5 mm, respectively. The three-dimensional numerical model of natural draft wet cooling tower is developed and validated by the published data, and the cooling performance with the novel water distribution mode is then obtained. The results show that the air velocity in the central area is significantly increased thus the velocity gets more uniformly distributed with the novel water distribution mode. Besides, the thermal performance in the external area is also improved thanks to the larger contact area between the cooling air and water droplets. By comparing the proposed water distribution with the conventional one, it can be found that the outlet water temperature decreases by up to 0.69 °C, and the tower with the proposed water distribution can achieve better heat and mass transfer performances with a lower air mass flow rate, which can be recommended in engineering applications for an ideal energy efficiency of cold end system.

유수율 제고를 위한 블록시스템 고립구축의 적용

In this study, the definition and standards of the block system, and the block isolation confirmation method were reviewed to establish an effective block system. The study target area was selected and a block system construction plan was established. First, for small blocks that cannot find unused pipelines connecting between small blocks, a method of operating small blocks by partially changing the small block boundaries planned in the drawing to match the site conditions can also be an effective method for block system management. The second is to secure water quality stability through flushing. Blocking the flow and changing the flow direction by operating the valve to confirm the small block isolation is causing red water and pollutants. First of all, it is necessary to examine the method of discharging the accumulated pollutants and red water in the pipe. Finally, Before the block isolation confirmation, the boundaries were not divided by drainage or unit area, and the drainage amount could not be analyzed because flow measurement facilities by small block were not equipped. However, after the block isolation was completed, it was possible to calculate the flow rate according to each small block by installing a flow meter at the inlet point. As a result of measuring the flow rate for each small block according to the block isolation confirmation, the SJ-3 small block had the highest average flow rate with 88.9 %, and the SJ-2 small block had the lowest average flow rate with 66.2 %. In addition, it is necessary to subdivide the water supply area for small blocks with low flow rate. It is judged that effective pipe network management will be possible if a small flow rate monitoring system is installed at the inlet point of the subdivided water supply area.

Preliminary Design of an Axial-Flow Turbine for Small-Scale Supercritical Organic Rankine Cycle

A small-scale organic Rankine cycle (ORC) with kW-class power output has a wide application prospect in industrial low-grade energy utilization. Increasing the expansion pressure ratio of small-scale ORC is an effective approach to improve the energy efficiency. However, there is a lack of suitable expander for small-scale ORC that can operate with a high efficiency under the condition of large expansion pressure ratio and small mass flow rate. Aiming at the design of high-efficiency axial-flow turbine in small ORC system, this paper investigates the performance of a kW-class axial-flow turbine and proposes a method for efficiency improvement. First, the preliminary design of an axial-flow turbine is conducted to optimize the geometric parameters and aerodynamic parameters. Then, the effects of tip clearance and trailing edge thickness on turbine performance are analyzed under design and off-design conditions. The results show that the efficiency of the two-stage or three-stage turbine is evidently better than that of the single-stage one. The output power and efficiency of the three-stage turbine are close to that of the two-stage turbine while the speed is lower. Meanwhile, the trailing edge loss and leakage loss can be significantly reduced via reducing the trailing edge thickness and tip clearance, and thus the turbine efficiency can be improved significantly. The estimated efficiency arrives at 0.82, which is 33% higher than that of the conventional turbine. Considering the limitation of turbine speed, three-stage axial-flow turbine is a feasible choice to improve turbine efficiency in a small-scale ORC.

Increased detector response in optical overfilled measurements due to gas lens formation by nitrogen flow through the entrance aperture

According to our experimental results, a nitrogen flow used to prevent dust and moisture entering a detector may influence measurements performed with trap detectors in overfilled conditions. A stable light source was measured with a wedged trap detector with 4 mm aperture, and the nitrogen flow rate was varied. The nitrogen flow was found to have the largest effect of up to 0.8% on the responsivity of the detector at around 1.0 l min−1 flow rate. The effect of nitrogen flow can be removed down to 0.02% by an added crossflow which removes the nitrogen out of the optical axis. In another experiment, the effect was removed almost completely by changing the flowing gas from nitrogen to synthetic dry air. We also present measurement results that indicate the responsivity changes with nitrogen to be smaller than 0.05% with underfilled beam geometry, even without the added crossflow. Based on simulations, the nitrogen flow through the detector forms a gradient-index type gas lens in front of the detector increasing the effective aperture area and thus the responsivity. In the underfilled measurement geometry there is no light close to the aperture edge which could be refracted inside the detector. Finally, we consider methods to ensure that the responsivity changes due to the gas flow remain below 0.05% in overfilled measurement geometry, without compromising the cleanliness of the detector with too small gas flow rate.

Numerical Study the Effect of Blade Number on Archimedes Screw Turbine Utilized in Pico-Hydro in Horizontal Position

Application of Archimedes Screw Turbine (AST) on pico-hydro has been very popular due to its capability to has a high torque with small flow rate and environmentally friendly to biotic of vicinity. This study uses CFD to investigate AST performance when the number of blades is varied. Independency of mesh is conducted to find an efficient number of mesh and it is obtained at 80 thousand mesh. Variation A, B and C respectively has 4,6, and 9 blades varied to analyze the correspondence with torque, pressure loss and velocity contour. The results show that with a higher number of blades would make higher torque. This is because energy extraction from water would be more efficient at higher number blade. However, the side back at such number would arise such as the higher-pressure loss. This is corresponding to energy balance where a high delta pressure is proportional with a generated energy turbine.

Thermal and mechanical performance of a hybrid printed circuit heat exchanger used for supercritical carbon dioxide Brayton cycle

Printed circuit heat exchangers with high heat transfer efficiency, small volume and high pressure resistance are usually chosen as a precooler for the supercritical carbon dioxide Brayton cycle. The supercritical carbon dioxide channel has high pressure and small mass flow rate, but the water channel has low pressure and large mass flow rate. The conventional printed circuit heat exchanger does not consider the difference between the two-side channels, and thus the weight is large. In this work, a diffusion-bonded hybrid printed circuit heat exchanger is put forward as the precooler used for supercritical carbon dioxide Brayton cycle. The new structure uses chemical-etching channels for the supercritical carbon dioxide side and plate-fin channels for the water side. Thermal design of the hybrid printed circuit heat exchanger based on a segmented Log-Mean Temperature Difference method is conducted. The results indicate that the hybrid printed circuit heat exchanger can effectively improve the heat transfer performance. Compared with the conventional printed circuit heat exchanger, the core volume of the optimized hybrid printed circuit heat exchanger is reduced by 49%, and the heat transfer rate per unit volume is increased by 145%. The optimized hybrid printed circuit heat exchanger in both channels can satisfy the mechanical stress requirement.