Low Mass Flow Research Articles

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.

Electron cyclotron resonance discharge enhancement in a cusped field thruster

Electron cyclotron resonance (ECR) is induced to enhance the discharging performance of a cusped field thruster, by feeding microwave into the channel. Due to a critical combination of ECR discharge and DC discharge, propellant utilization is significantly improved, with varying degrees of impact on other parameters. The DC mode, the ECR mode and the combined mode are achieved and analyzed respectively. A Faraday probe and a retarding potential analyzer are employed to diagnose the plume plasma, and approximate calculated thrust is given. Moreover, high frequency electric field simulation is carried out to indicate the microwave feeding process in different mass flow rate conditions, making use of a particle-in-cell simulation result. The result shows that the ECR enhancement is particularly effectual in low mass flow conditions, and the special plasma distribution in the cusped magnetic field leads to the propellant utilization trend in the combined mode.

Circumferential Variation of Noise at the Blade-Pass Frequency in a Turbocharger Compressor with Ported Shroud

<div class="section abstract"><div class="htmlview paragraph">The ported shroud casing treatment for turbocharger compressors offers a wider operating flow range, elevated boost pressures at low compressor mass flow rates, and reduced broadband whoosh noise in spark-ignition internal combustion engine applications. However, the casing treatment elevates tonal noise at the blade-pass frequency (BPF). Typical rotational speeds of compressors employed in practice push BPF noise to high frequencies, which then promote multi-dimensional acoustic wave propagation within the compressor ducting. As a result, in-duct acoustic measurements become sensitive to the angular location of pressure transducers on the duct wall. The present work utilizes a steady-flow turbocharger gas stand featuring a unique rotating compressor inlet duct to quantify the variation of noise measured around the duct at different angular positions. The acoustic pressure transducers installed on the rotating duct record time-resolved in-duct acoustic pressure at different azimuthal locations while the compressor is held at a steady operating point. Acoustic measurements are performed across the flow range of a ported shroud compressor at three different rotational speeds. A comparison of sound pressure levels measured at different azimuthal locations reveals the significant contribution of high-frequency BPF noise to the variation in the acoustic pressure around the duct.</div></div>

Study of Pressure Drops and Heat Transfer of Nonequilibrial Two-Phase Flows

Currently, there are no universal methods for calculating the heat transfer and pressure drop for a wide range of two-phase flow parameters in mini-channels due to changes in the void fraction and flow regime. Many experimental studies have been carried out, and narrow-range calculation methods have been developed. With increasing pressure, it becomes possible to expand the range of parameters for applying reliable calculation methods as a result of changes in the flow regime. This paper provides an overview of methods for calculating the pressure drops and heat transfer of two-phase flows in small-diameter channels and presents a comparison of calculation methods. For conditions of high reduced pressures pr = p/pcr ≈ 0.4 ÷ 0.6, the results of own experimental studies of pressure drops and flow boiling heat transfer of freons in the region of low and high mass flow rates (G = 200–2000 kg/m2 s) are presented. A description of the experimental stand is given, and a comparison of own experimental data with those obtained using the most reliable calculated relations is carried out.

Directional Effects of Effusion Cooling on the Cooling Film Effectiveness

Effusion cooling in gas turbine combustors is complicated by swirling flows for anchoring flame, whereas miniature effusion cooling jets are highly directional. This work aims at understanding the directional effects of effusion cooling by studying the effectiveness of cooling film as a function of the offset flow angle β between the main flow and effusion cooling jets. The directional effects of effusion cooling on the cooling film effectiveness are investigated at offset angles of 0, 30, 60, and 90 deg. The cooling film effectiveness is determined by invoking the heat/mass transfer analogy using pressure-sensitive paint that is sensitive to the oxygen partial pressure at the paint surface. Experimental results indicate that offset angles have a strong effect on the cooling film effectiveness. At lower coolant mass flow rates, an offset angle increases the uniformness of the cooling film because the flows from each individual hole coalesce together to form the protective film. At higher coolant mass flow rates, the cooling film effectiveness is less sensitive to the offset angles . This is believed to be caused by the coolant flow lifting off the surface and dissipating more easily in the mainstream flow.

A new design of launcher mirror for EAST electron cyclotron resonance heating system

Significant progress has been achieved on EAST tokamak with the electron cyclotron resonance heating (ECRH), but the maintenance current EC launcher is very complicated after each campaign period. In order to reduce the maintenance frequency and cut down the maintenance cost, a new launcher mirror unit was developed based on adjusting the water cooling pipe lines layout and designing the detachable mirror. It improves the bending conditions and extends the life of the metal hoses effectively. Meanwhile, in situ replacement of the mirror reflector inside the device could be realized with the new design. The new launcher mirror employs a type of dot-matrix water cooling channel structure. It is demonstrated that the cooling design satisfies ECRH operating requirement with lower mass flow by fluid-thermal-solid coupling analysis.

Characteristics of medium-low rank coal blending on performance and efficiency steam power plant

Blending medium rank coal (MRC) with low-rank coal (LRC) is carried out to meet the caloric content which follows the boiler specifications. Negative effects of blending with LRC are environmental, specific fuel consumption (SFC), and efficiency issues. The purpose of this research is the optimum blending composition of LRC and MRC coal, which can be used by power plants. The research methodology used is to simulate coal blending with MRC percentage in the MRC-LRC from 0, 20%, 40%, 50%, 60%, 80%, and 100%. The type of coal used is categorized in MRC1, MRC2, LRC1, LRC2, LRC3. The combination used is MRC1-LRC1, MRC1-LRC2, and MRC2-LRC3. From the analysis obtained, if the MRC percentage in MRC-LRC blending increases, losses will decrease mass flow will decrease, SFC will decrease, and boiler efficiency will increase. Blending MRC2-LRC3 has the lowest losses, low mass flow with an average of 298.78 ton/h, low SFC with an average of 0.466 kg/kWh, and boiler efficiency between 84.85% to 85.71 %. Blending MRC2-LRC3 is most recommended for use in combustion plant with a minimum MRC2 percentage of 50%.

Effects of vapor duct thickness on the capillary blocking and thermal performance of ultra-thin vapor chambers under natural convection cooling

This work presents the performance test results for novel two-piece ultra-thin vapor chambers with a thickness (t) of 0.38 mm, 0.48 mm and 0.58 mm, having a vapor duct thickness (h) of 0.15 mm, 0.25 mm, and 0.35 mm, respectively. These ultra-thin vapor chambers simply comprise a top plate etched with arrayed pillars as the condenser and a mesh-wicked bottom plate as the evaporator and liquid path. The inner surface of the top plate contains cross parallel vapor ducts configured by aligned pillars for vapor spreading all over the vapor chamber. The pillars provide strong structural support, an enlarged condensation area, and a direct shortcut for liquid return. Compared to traditional vapor chambers, the novel ultra-thin vapor chambers have the advantages of simpler structure, easier manufacturing process, lower cost, superior anti-compression ability, and better thermal performance. They keep excellent structural integrity at a footprint of 140 mm × 80 mm. Cooled by natural convection, the vapor chamber resistances (Rvcs) are measured for different hs at different filling ratios (γs). Capillary blocking is observed for h = 0.15 mm and 0.25 mm at γ larger than 1.0, but it does not appear for h = 0.35 mm even with considerable over-charge. Although capillary blocking may slightly raise Rvcs, it can be avoided by suitable under-charging. Except for cases with significant over-charge, Rvcs are similar for h of 0.15 mm to 0.35 mm at heat loads less than 15 W due to the low vapor resistances at low mass flow rates. At heat loads of about 14 W, the minimum Rvcs of unblocked ultra-thin vapor chambers are about 0.25 K/W to 0.3 K/W, greatly superior to the value of 0.88 K/W of a 0.5 mm-thick copper plate.

Numerical Study of the Improvement in Stability and Performance by Use of a Partial Vaned Diffuser for a Centrifugal Compressor Stage

The influence of different diffuser configurations on the flow stability and aerodynamic performance of a centrifugal compressor stage with a mass flow coefficient of 0.196 is numerically investigated. Research results show that the performance of a traditional full-height vaned diffuser (TVD) deteriorates rapidly, and a shroud-side partial vaned diffuser (SVD) displays better adaptability in off-design conditions. SVD can suppress the development of vortices generating at the diffuser leading-edge. Therefore, it can reduce the flow loss inside the stage and improve the flow stability of the stage at low mass flow rates. The unsteady analysis for TVD and SVD shows that the stall cell propagates at about 35.7% of impeller rotational speed in the semi-vaneless space and diffuser passages. Furthermore, the internal flow in TVD and SVD is studied by employing the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods. The flow loss and instability mechanism in the stage are consequently revealed more comprehensively.

Numerical simulation of thermal–structural behaviour of liquid helium tank during filling process

In a large-scale helium cryogenic system, a tank is used to store liquid helium. To predict the cooling behaviour and thermal performance of the tank during liquid helium filling, a simple and efficient computational fluid dynamics methodology was used. The phase transition and temperature distribution in the tank were simulated through a two-dimensional thermal–fluid–structure coupling model. Based on the temperature data in the simulation, the stress and deformation on the wall surface were calculated using a finite element model. Liquid helium filling processes with different mass flow rates are analysed and compared, and the impacts on phase transition, liquid temperature, and thermal effects in the tank wall are discussed. The results show that the height of the liquid filling level has a significant influence on the boiling evaporation rate, the degree of temperature stratification, and the flow structure. The gas disturbance at the bottom of the tank is obvious, and the gas–liquid interface is in an unbalanced state, especially with a low liquid level. The simulation data indicate that the mass flow rate has a greater impact on the initial stage of filling. Oscillation of the gas–liquid interface is more evident with a lower mass flow rate. With an increase in the mass flow rate, temperature stratification becomes increasingly severe. According to the thermal calculation data, a filling rate that is too high or too low can lead to higher stress and larger deformation on the tank wall.

Net-exergetic, hydraulic and thermal optimization of coaxial heat exchangers using fixed flow conditions instead of fixed flow rates

Previous studies optimized the dimensions of coaxial heat exchangers using constant mass flow rates as a boundary condition. They show a thermal optimal circular ring width of nearly zero. Hydraulically optimal is an inner to outer pipe radius ratio of 0.65 for turbulent and 0.68 for laminar flow types. In contrast, in this study, flow conditions in the circular ring are kept constant (a set of fixed Reynolds numbers) during optimization. This approach ensures fixed flow conditions and prevents inappropriately high or low mass flow rates. The optimization is carried out for three objectives: Maximum energy gain, minimum hydraulic effort and eventually optimum net-exergy balance. The optimization changes the inner pipe radius and mass flow rate but not the Reynolds number of the circular ring. The thermal calculations base on Hellström’s borehole resistance and the hydraulic optimization on individually calculated linear loss of head coefficients. Increasing the inner pipe radius results in decreased hydraulic losses in the inner pipe but increased losses in the circular ring. The net-exergy difference is a key performance indicator and combines thermal and hydraulic calculations. It is the difference between thermal exergy flux and hydraulic effort. The Reynolds number in the circular ring is instead of the mass flow rate constant during all optimizations. The result from a thermal perspective is an optimal width of the circular ring of nearly zero. The hydraulically optimal inner pipe radius is 54% of the outer pipe radius for laminar flow and 60% for turbulent flow scenarios. Net-exergetic optimization shows a predominant influence of hydraulic losses, especially for small temperature gains. The exact result depends on the earth’s thermal properties and the flow type. Conclusively, coaxial geothermal probes’ design should focus on the hydraulic optimum and take the thermal optimum as a secondary criterion due to the dominating hydraulics.

CENTRIFUGAL COMPRESSOR PERFORMANCE MAPS TREATMENT FOR INTERNAL COMBUSTION ENGINES OPERATING CYCLE SIMULATION

Simulation of the supercharged internal combustion engines operation cycle is impossible without correct estimation of the supercharger operating parameters. Standard approach is to use specially prepared performance maps of compressor and turbine of the turbocharger, which are based on the experimental (or manufacturer’s) raw data. Centrifugal compressor performance maps interpolation, extrapolation and treatment provides challenging requirements as it is important to get correct simulation under such special conditions as compressor choke, rotating stall and pumping surge. At the same time it’s important to obtain the fast and stable calculations of the engine’s operating cycle. Blitz-PRO – online internal combustion engines operating cycle simulation service – offers supercharger performance maps preprocessing and implementation. It provides three different modes of compressor surge consideration during calculations: 1) full-scale surge mode using Moore-Greitzer approach; 2) mild surge mode with flexible adjustment; 3) “stable” mode, when the surge is neglected and the compressor constant-speed lines are extended from the rotating stall point to the lower mass flow region with the hyperbolic equation. Using the MAN 8G70ME-E engine 12140 kW, 82 rpm operating point as an example, the calculation results are compared for three modes of compressor surge consideration. The “stable” mode provides the fastest and the most stable calculations, while the calculations under the full-scale surge mode could generate the numerical (nonphysical) instability of calculations, which are caused by the high sensitivity of the two-stroke engines to the gas exchange processes as it is shown. The mild surge mode provides fast and stable enough calculation with the surge consideration ability, which could be assumed as the best solution for the given example. The researcher should choose between provided three modes of the centrifugal compressor surge consideration according to the calculations tasks, preferring “stable” mode for initial model setup and mild surge mode for the surge probability check, while the accurate compressor surge simulation needs further development.

Flow boiling transient critical heat flux tests with stainless steel and FeCrAl: Transient correlation implementation, model calibration, and sensitivity analysis

Steady-state and transient internal-flow critical heat flux (CHF) experiments were carried out under two flow conditions, at atmospheric pressure, on 316 stainless steel (316-SS) and iron-chromium-aluminum (FeCrAl) tubes. Slow power transients at a low mass flow (300kg/m2−s) with very low subcooling (Xe=−0.0054) generated premature CHFs, which were prevented by faster power transients and high mass flow (1,000kg/m2−s). The measured transient CHFs increased linearly with increasing power transients, compared with the steady-state CHF. Yet, the wall superheat at the CHFs decreased with faster power transients. Transient CHF correlations highlighting heterogeneous spontaneous nucleation were calibrated to the measured CHFs and compared with other existing correlations. Transient CHF multipliers were acquired from pool and flow boiling empirical CHF correlations that were generated. The multipliers were applied to the RELAP5-3D nuclear system code to analyze the discrepancies between the measured data and the predicted CHF and post-CHF behavior, which improved peak cladding temperature predictions by 24.6%. A variance-based global sensitivity study perturbing the experimental uncertainties and cladding material thermal properties showed the diminishing influence of flow boiling heat transfer with increasing power transients, highlighting the significance of the volumetric heat capacity for the cladding integrity during transients. Transient CHF correlations were applied to the most limiting design basis accident: a hot-zero-power reactivity-initiated accident on a generic pressurized water reactor RELAP5-3D model.

Numerical study of slot-die coating on film formation for different die lip configurations

ABSTRACT Slot-die coating is a promising technology for the development of integrated circuits. In this study, we used ANSYS Fluent, to investigate slot-die coating for different die lip configurations. We compared the coating window results with published experimental data. The results were consistent at low coater speeds and mass flow rates; however, we observed a large discrepancy at the highest speeds and pump flow rates. This could be because the two-dimensional model fails to predicta realistic boundary condition in the spanwise direction. We also implemented a three-dimensional model to predict the uniformity of the slot-die coating for different coating gaps (0.25, 0.3, and 0.35 mm). The results show that the 0.25 mm gap results in the smallest thickness, while the 0.3 mm gap has the best uniformity. We considered different die lip configurations. When the pump flow rate was 1.904 g/s, the coater speed was 0.1 m/s, and the die lip was 0.5 mm upstream and 0.25 mm downstream, the film thickness was 12.2 μm. Under the same conditions, when the die lips were 0.25 mm upstream and 0.5 mm downstream, the film thickness was nearly the same at around 13.2 μm. The pressure gradient plays an important role in a faster coating process.

Preliminary Experimental Characterization of a Microwave Discharge Cusped Field Thruster

The cusped field thruster has become a candidate thruster in next-generation space science missions due to its low complexity and potential long life over a wide range of thrust conditions. However, the reduction of propellant utilization and, as a result, the total performance degradation has emerged in works on cusped field thruster downscaling. In this work, a coaxial resonator is utilized to feed the microwave (MW) into the near-anode region of the cusped field thruster, which contributes to ionization at a relatively low mass flow rate. With a LaB6 thermionic emission cathode, the thruster can operate in two modes: MW excited mode and the MW-DC mixed mode. The results show that the thruster can operate with a minimum mass flow rate minimum of 0.004 mg/s and achieves a continuously adjustable thrust from 1.9 μN to 30.8 μN. The performance variation characteristics over the three control parameters (anode voltage, mass flow rate, and MW power) are considered. Additionally, a plume plasma diagnosis is performed with a Faraday probe and a retarding potential analyzer to analyze the discharge performance and plasma characteristics with different operating conditions.

Simulation investigation of inductively coupled plasma generator for atmosphere-breathing electric propulsion system

An atmosphere-breathing electric propulsion system (ABEP) uses the rarefied atmospheric molecules as the propellant for the electric thruster. In the best case, it can allow spacecraft (S/C) complete a long-time mission in very low Earth orbit (VLEO) without carrying any propellant. However, previous studies have shown that the thruster lifetime is limited by electrode erosion due to the aggressive gases, such as the atomic oxygen in VLEO. This paper design and analyse an inductively coupled plasma generator (ICPG) for ABEP system, and it is electrodeless to avoid electrode erosion. Based on the COMSOL Multiphysics, the simulation model of ICPG is established and analyzed. Multiple physical fields are coupled in this model including plasma, magnetic field, and flow field. The simulation model is verified by the experimental results of the literature. Driving by the radio frequency power, the plasma power distribution and thrust estimation of the ICPG under different input conditions (different drive powers and different operation gases) can be obtained. The results show that the ICPG may have a better performance under the condition of low mass flow and high drive power to compensate the drag. The gas species and their proportions have also great influence on the behaviours of ICPG. This paper can provide a convenient and efficient method to study the ICPG products for ABEP.

Experimental Performance of a Membrane Desorber Operating under Simulated Warm Weather Condensation Temperatures.

In absorption systems using the aqueous lithium bromide mixture, the Coefficient of Performance is affected by the desorber. The main function of this component is to separate the refrigerant fluid from the working mixture. In conventional boiling desorbers, constant heat flux and vacuum pressure conditions are necessary to carry out the desorption process, and usually, the absorbers are heavy and bulky; thus, they are not suitable in compact systems. In this study, a membrane desorber was evaluated, operating at atmospheric pressure conditions with a water/lithium bromide solution with a concentration of 49.6% w/w. The effects of the solution temperature, solution mass flow, and condensation temperature on the desorption rate were analyzed. The maximum desorption rate value was 6.1 kg/m2h with the following operation conditions: the solution temperature at 95.2 °C, the solution mass flow at 4.00 × 10−2 kg/s, and the cooling water temperature at 30.1 °C. On the other hand, the minimum value was 1.1 kg/m2h with the solution temperature at 80.2 °C, the solution mass flow at 2.50 × 10−2 kg/s, and the cooling water temperature at 45.1 °C. The thermal energy efficiency, defined as the ratio between the thermal energy used to evaporate the refrigerant fluid with respect to the total thermal energy entering the membrane desorber, varied from 0.08 to 0.30. According to the results, a high solution mass flow, a high solution temperature, and a low condensation temperature lead to an increase in the desorption rate; however, a low solution mass flow enhanced the thermal energy efficiency. The proposed membrane desorber could replace a conventional boiling desorber, especially in absorption cooling systems that operate at high condensation temperatures as in warm weather regions.

Single-phase density wave oscillation -A new mechanism of flow instability in inverted U-type steam generator

Flow reversal under low mass flow conditions is harmful to the performance of inverted U-type steam generator (UTSG) and existing research attributes the phenomenon to be flow excursion instability. In this paper, a new mechanism of flow instability that could also induce flow reversal, namely single-phase density wave oscillation (DWO), was analyzed based on numerical methods. As a primary study, the analysis was done in parallel channels consisted of two identical inverted U-type tubes. It was found that for UTSG at low mass flow conditions, the compressibility of single-phase flow would be a key factor to trigger the positive feedback of mass flow-temperature-pressure drop in DWO. Single-phase DWO should result in premature of flow reversal compared to that caused by flow excursion (FE), and analysis showed that it could occur only if the primary side temperature difference and average temperature of UTSG were both large enough, and for large-scale UTSG DWO would disappear due to the increase of total pressure drop. Parameter effects on single-phase DWO in UTSG were also carefully studied and it was found that the trends could be well explained by the variation of system compressibility.

Experimental and Numerical Assessment of the Impact of an Integrated Active Pre-Swirl Generator on Turbocharger Compressor Performance and Operating Range

The implementation of increasingly stringent emissions and efficiency targets has seen engine downsizing and other complementary technologies increase in prevalence throughout the automotive sector. In order to facilitate ongoing improvements associated with the use of these strategies, delivering enhancements to the performance and stability of the turbocharger compressor when operating at low mass flow rates is of paramount importance. In spite of this, a few concepts (either active or passive) targeting such aims have successfully transitioned into use in automotive turbochargers, due primarily to the requirement for a very wide compressor-operating range. In order to overcome the operational limitations associated with existing pre-swirl generation devices such as inlet guide vanes, this study developed a concept comprising of an electrically driven axial fan mounted upstream of a standard automotive turbocharger centrifugal compressor. Rather than targeting a direct contribution to compressor boost pressure, the fan was designed to act as a variable pre-swirl generation device capable of being operated completely independently of the centrifugal impeller. It was envisioned that this architecture would allow efficient generation of the large pre-swirl angles needed for compressor surge margin extension and efficiency enhancement at low mass flow rate-operating points, while also facilitating the delivery of zero pre-swirl at higher mass flow rates to ensure no detrimental impact on performance at the rated power point of the engine. Having progressed through 1-D and 3-D aerodynamic modelling phases to understand the potential of the system, detailed component design and hardware manufacture were completed to enable an extensive experimental test campaign to be conducted. The experimental results were scrutinized to validate the numerical findings and to test the surge margin extension potential of the device. Compressor efficiency improvements of up to 3.0% pts were witnessed at the target-operating conditions.

Experiments of Transpiration Cooling Inspired Panel Cooling on a Turbine Blade Yielding Film Effectiveness Levels over 95%

Panels were tested at different locations around the turbine blade, on both suction and pressure surfaces. Three different surface porosities were also tested. Results demonstrated that the approach can be very successful with high levels of film cooling effectiveness, exceeding 95%, achieved using low coolant mass flow rates. Increasing the surface porosity also proved to be an important parameter in the panel’s performance. Additionally, staggering the film holes lead to significant positive interactions between individual films, resulting in much improved panel performance.