| 1. |
- Guo, Qiang, et al.
(författare)
-
An experimental study on quenching of metallic spheres in seawater
- 2023
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 148
-
Tidskriftsartikel (refereegranskat)abstract
- Motivated by quenching of melt droplets and debris particles in seawater during a hypothetical severe accident of light water reactors (LWRs), the quenching process of stainless-steel spheres in a seawater pool was investigated in the present study. The polished spheres were pre-heated up to 1000celcius in an induction furnace with inserted atmosphere, and then immersed into the subcooled water pool in a chamber made of transparent quartz. A thermocouple was embedded in the center of the sphere to measure the history of the sphere's temperature, while a high-speed camera was employed to record the quenching process and vapor film dynamics. Quantitative data, e.g. film thickness and oscillation, of the vapor film evolution during the quenching process were obtained through an image processing program developed on the MATLAB platform.The experimental results indicated that the quenching rate was higher in seawater than in deionized water, and the vapor film collapsed at a temperature higher than the Leidenfrost temperature in deionized water. The trend appeared more significant with increasing subcooling of water. The comparison of the quenched spheres suggested the surface of the sphere in seawater achieved higher degree of discoloration and roughening than that in deionized water, probably due to the additives of salt which change water properties. The image processing and analysis revealed that the vapor film had different thickness profile along the upper and lower hemispheres, and the averaged film thickness was smaller in seawater than in de-ionized water during the stage of stable film boiling. The vapor film was thinning and oscillating with time, and its fluctuations appeared different frequencies and amplitudes at the upper and lower locations, which may explain the mechanism of the earlier collapse of vapor film in the quenching process of a high-temperature sphere in seawater.
|
|
| 2. |
- Lundström, Hans
(författare)
-
Investigation of heat transfer from thin wires in air and a new method for temperature correction of hot-wire anemometers
- 2021
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 128
-
Tidskriftsartikel (refereegranskat)abstract
- Variations in fluid temperature are the most important error source in hot-wire anemometer measurements and must sometimes be compensated for. Although many temperature compensation schemes have been proposed over the years, no consensus seems to be reached regarding correction methods. In this paper new precision measurements on heat transfer from heated wires have confirmed that, provided the fluid properties are evaluated at the mean temperature between the air and sensor temperatures, the Nusselt number is a consistent function of the Reynolds number without any further corrections. Based on this knowledge a new calibration function which accounts for temperature dependencies in fluid properties is proposed. The aim has been to come up with a calibration function that is useful in practical anemometry. The only parameters in the function are air- and sensor temperatures. Velocity calibration is only necessary at one air temperature because temperature dependencies on conductivity and viscosity are incorporated in the calibration function.
|
|
| 3. |
- Lundström, Hans, et al.
(författare)
-
Radiation influence on indoor air temperature sensors : Experimental evaluation of measurement errors and improvement methods
- 2020
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; 115
-
Tidskriftsartikel (refereegranskat)abstract
- Radiation influence on commonly used temperature sensors for measuring indoor air temperatures can be significant, especially at the typically low air velocities occurring indoors. Conceptually, a physical sensor may not read the true air temperature, it only reads its own temperature, and, being a solid body, it will exchange energy with the surrounding surfaces (walls, windows etc.) through radiation. In the present study, radiation influence on indoor air temperature measurements was investigated experimentally and errors were quantified in simple terms. Measures to reduce the impact on some common temperature sensors were explored. A special test rig was built to simulate typical airflow and radiation environments indoors. It is suggested that the radiation impact on a temperature sensor is quantified by a radiation sensitivity factor defined as RSF = hrad/hconv, where hrad and hconv are heat transfer coefficients for radiation and convection, respectively. As this definition infers, the radiation sensitivity is dependent on size, geometry and emissivity of the temperature sensor. The radiation sensitivity factor, thus being unique for each type of sensor, was measured for some common types of thermistors and thermocouples. It is demonstrated that radiation errors may be reduced by 60 – 80 % on thermistors by reducing their emissivity through gold sputtering, and on thermocouples by stripping the insulation at the outermost part of their sensor leads.
|
|
| 4. |
- Sanmiguel Vila, C., et al.
(författare)
-
Experimental realisation of near-equilibrium adverse-pressure-gradient turbulent boundary layers
- 2020
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier Inc.. - 0894-1777 .- 1879-2286. ; 112
-
Tidskriftsartikel (refereegranskat)abstract
- A new experimental database of adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) obtained through hot-wire anemometry and oil-film interferometry covering a momentum–loss Reynolds number 450<Reθ<23450 and Clauser pressure-gradient-parameter range up to β≈2.4 is presented. Both increasing and approximately constant β distributions with the same upstream history are characterised. Turbulence statistics are compared among the different pressure-gradient distributions with additional numerical and experimental zero-pressure-gradient (ZPG) TBL data. Cases at approximately constant β, which can be considered as canonical representations of the boundary layer under a certain pressure-gradient magnitude, exhibit skin-friction and shape-factor curves consistent with the ones proposed by Vinuesa et al. (2017). These curves show a similar scaling behaviour as those proposed by Nagib et al. (2007) for ZPG TBLs. The pre-multiplied power-spectral density is employed to study the differences in the large-scale energy content throughout the boundary layer. Two different large-scale phenomena are identified, the first one related to the pressure gradient and the second one (also present in high-Re ZPG TBLs) due to the Reynolds number. Recently proposed scaling laws by Kitsios et al. (2016) and Maciel et al. (2018) are tested over a wider Reynolds-number range and for different β cases. The mean velocity and streamwise velocity fluctuation profiles are found to be dependent on the upstream development. The mean velocity profile is found to be self-similar only in the outer region, in agreement with classical theory. The mean and higher-order statistics of the new APG TBL database are made available under www.flow.kth.se.
|
|
| 5. |
- Yang, Yanjie, et al.
(författare)
-
Transport dynamics of droplet impact on the wedge-patterned biphilic surface
- 2020
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 113
-
Tidskriftsartikel (refereegranskat)abstract
- Droplet impact on biphilic surfaces with a wettability contrast has been intensively studied in recent years. In this work the effects of tilting and apex angles on droplet transport dynamics after impacting on a wedge-patterned biphilic surface at low Weber numbers were investigated experimentally. The biphilic surface was fabricated by applying a hydrophobic polymer coating on a bare silicon surface. According to the experimental results, a larger apex angle below 67.4° can accelerate the droplet effectively at first. Then the friction force controls the droplet movement and reduces the speed. The tilting angle along the hydrophilic direction activates the droplet. If the gravity component is opposite to the hydrophilic direction and the tilting angle is over 15°, the droplet can hardly move toward the hydrophilic area. By modeling the hydrodynamics of the droplet movement after impact on a biphilic surface with assumptions of no evaporation, no Marangoni effect, negligible dynamic contact angle variation and negligible rotation effect, the surface tension values versus the position at different apex angles are derived. The predicted position versus time trends agree well with the experimental data. This study aims to provide a better understanding of the mechanisms of droplet hydrodynamics on wedge-patterned biphilic surfaces at low Weber numbers.
|
|
| 6. |
- Zheng, Xiaobo, et al.
(författare)
-
Inter-scale interaction in pipe flows at high Reynolds numbers
- 2022
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777 .- 1879-2286. ; 131
-
Tidskriftsartikel (refereegranskat)abstract
- Hot-wire measurements of the streamwise velocity are conducted in the large-scale pipe-flow facility CICLoPE in the friction Reynolds number range of 7800 <= Re-tau < 40000. Measurements are performed both with a rake of five synchronised probes arranged at different radial locations, and through a radial scan with a single wire traversing the whole pipe radius. Correlation-based analysis is used to extract features of inter-scale modulation in turbulent pipe flows. The Re-tau-independence of geometric features is shown with the outer scaling. Very-large-scale motions keep the vertical coherence to the wall through the whole pipe radius, while for the large-scale motions, the local coherence gradually become isotropic as the structure centre moves far away from the wall. From results obtained with the one-point amplitude modulation (AM) correlation function map, the AM effect is characterised by positive correlations observed in the inner region, while an opposite AM effect characterised by negative correlations is observed in the outer region. The strongest AM effect (with the maximum correlations) and the zero net-modulation (with zero correlations) show that the phase difference between large and small scales has a linear relation with the logarithm of the outer-scaled wall distance, but the strongest opposite effect (with the negative maximum correlations) behaves with the phase difference independent of the outer scaled wall-normal distance. Two-point AM coefficient maps, which give richer spatial information in the outer region than the one-point map, present the Re-tau-independencefor AM and opposite effects within the present Re-tau range. In addition, the relative variation of the two effects in the coexisting wall-normal range is characterised by identifying the maximum gradient of the one-point AM coefficient and the peak-to-peak value of the two-point AM coefficient map.
|
|
| 7. |
- Li, Haohan, et al.
(författare)
-
Investigation of evaporating sprays in a medium speed marine engine
- 2021
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 121
-
Tidskriftsartikel (refereegranskat)abstract
- The understanding of diesel sprays is very important to enable a better and cleaner marine engine design, but unfortunately little knowledge is openly available on marine engine fuel sprays. In this paper, evaporating sprays for medium speed marine engines were studied in a constant volume combustion chamber by performing optical measurements through Schlieren and Mie diagnostic techniques. The effects of ambient gas temperature and ambient gas density on vapor and liquid penetration were investigated by changing the target condition in the combustion chamber. A comparative study of two injectors with different nozzle diameters (0.38 mm and 0.44 mm) was also carried out at ambient density of 22.5 kg/m3. Some empirical correlations of spray penetration have been modified to fit the spray measurement data. Due to the transient characteristics of the pump-line-nozzle injection system, a time-dependent injection pressure profile is suggested for calculation of spray penetration. The spray tip penetration at large distance under low density (7.6 and 15.2 kg/m3) conditions is expected to be proportional to t2/3, which is supported by the model considering spray-induced gas turbulence effect. The t1/2 law, where turbulence is not taken into account, is still valid under high density (22.5 kg/m3) conditions with higher engine load. The comparison of two models demonstrates that the effect of gas turbulence is influenced by the ambient gas density and engine load.
|
|
| 8. |
- Li, Xingchen, et al.
(författare)
-
Breakup dynamics of gas-liquid interface during Taylor bubble formation in a microchannel flow-focusing device
- 2020
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 113
-
Tidskriftsartikel (refereegranskat)abstract
- This work aims to investigate the breakup dynamics of the gas-liquid interface during bubble formation in a microchannel flow-focusing device. An interface tracking method is developed to capture the profiles of the gaseous thread evolution. The results show that the pinch-off period can be further divided into a liquid squeezing stage and a free pinch-off stage in both the radial and axial directions. The time domain criterion between these two stages in a low viscous liquid, with Ohnesorge numbers Oh≪1, is proved to be shorter than the capillary time. The effects of surface tension, viscosity and gas inertial force exerting on the interface during the free pinch-off stage are proved similar to those in a quiescent liquid pool. The power law of the minimum diameter at the gaseous thread to the pinch-off remaining time in the present experiments agrees with previous studies in both ranges (1/3 to 1/2) and tendency.
|
|
| 9. |
- Pignatelli, F., et al.
(författare)
-
Impact of pilot flame and hydrogen enrichment on turbulent methane/hydrogen/air swirling premixed flames in a model gas turbine combustor
- 2024
-
Ingår i: Experimental Thermal and Fluid Science. - 0894-1777. ; 152
-
Tidskriftsartikel (refereegranskat)abstract
- This work investigates the impact of pilot flame and fuel composition on the structures and stabilization of swirling turbulent premixed methane/hydrogen/air flames in a lab-scale gas turbine model combustor. Simultaneous measurements of the velocity field and OH radicals distribution in the combustor were conducted using particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF) methods, respectively. Flames under stable and close to lean blow-off (LBO) conditions were studied for two fuel mixtures, with a hydrogen mole ratio of 0 and 50 % in the hydrogen/methane mixture, respectively. The studied flames were at a constant Reynolds number of 20,000 with different equivalence ratios. Two pilot-to-global fuel ratios were investigated (2 % and 6 %) while keeping the pilot-to-global air ratio constant at 2 %. Data for non-piloted flames were also acquired for comparison. The pilot flames were shown to extend the operability range. The LBO equivalence ratio of the main flame decreased with increasing fuel mass flow rate in the pilot flames due to the increased amount of hot gases with high concentrations of OH radicals in the outer recirculation zone (ORZ), which significantly enhanced the stabilization of the main flame. The stable flame reaction zone was in the high-speed shear layer between the ORZ and the inner recirculation zone (IRZ). When approaching LBO, the reaction zone was pushed downstream to the IRZ and subsequently decreased the size of IRZ, indicating a strong flow/flame interaction. Hydrogen enrichment was shown to reduce the LBO equivalence ratio. When close to LBO, the OH radicals in the hydrogen-enriched flames were observed in isolated pockets due to differential diffusion, which enhanced resilience to LBO. The flame front curvature, mean progress variable, and flame surface density were calculated from the acquired OH-PLIF data to quantify the impact of fuel composition and pilot flames on the flame structures.
|
|
| 10. |
- Zhang, Bo lun, et al.
(författare)
-
Experimental study on film cooling performance of a turbine blade tip with a trapezoidal slot cooling scheme in transonic flow using PSP technique
- 2022
-
Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 130
-
Forskningsöversikt (refereegranskat)abstract
- A turbine blade tip with a trapezoidal slot cooling scheme is first proposed in the current paper. The film cooling characteristics of a turbine blade tip with a trapezoidal slot scheme are experimentally studied by the pressure sensitive paint (PSP) technique in transonic flow. Varieties of density ratio, tip clearance gap and position of the trapezoidal slot scheme are selected as investigation parameters. The mainstream Reynolds number is 370,000 based on the blade axial chord length. The cascade exit Mach numbers are is 1.05. The results indicate that increasing the density ratio has a positive effect on the film cooling effectiveness of the whole blade tip with a pressure side trapezoidal slot scheme at a small tip clearance gap, whereas it decreases the film cooling effectiveness upstream of the blade tip at the large tip clearance gap. For the blade tip with a pressure side trapezoidal slot scheme, the film cooling effectiveness of the tip clearance gap of 1.5% near the trailing edge is higher than that of the other tip clearance gaps at the same density ratio condition. Changing the trapezoidal slot position from the tip pressure side to the tip midline increases the film cooling effectiveness near the trailing edge.
|
|
| 11. |
|
|
