Nucleate pool boiling heat transfer of SES36 fluid on nanoporous surfaces obtained by electrophoretic deposition of Al2O3

With the aim of enhancing pool boiling heat transfer coefficient (HTC), the nucleate boiling performance of nanoporous surfaces obtained by an electrophoretic deposition (EPD) method is evaluated in this paper, with SES36 as the boiling fluid. A pool boiling experimental apparatus and procedure are described. Three kinds of experiment have been performed: (i) smooth stainless steel (SS) surface with pure SES36, providing the baseline; (ii) smooth SS surface with boiling nanofluid consisting of 0.5, 1 and 2 wt% Al2O3 suspended in SES36; (iii) nanoporous surfaces, of SS coated by EPD in procedures using 0.5, 1 and 2wt% concentrations of Al2O3, with pure SES36 as the boiling fluid. In (ii), the results show that the HTC of the smooth SS surface deteriorated with increasing concentration of Al2O3. In (iii), however, the HTC increased by approximately 6.2%, 30.5% and 76.9% for surfaces prepared with suspensions containing 0.5, 1 and 2 wt% Al2O3 respectively under the heat flux of 90 kW/m2, compared with the baseline of the smooth surface in (i). The boiling behaviors are related to the modified surface micro-morphology due to the deposition of nanoparticles, as visualised by scanning electron microscopy (SEM). The maximum active nucleation site density was about 2.6×105 sites/m2 for the 2 wt% EPD surface under 94 kW/m2, which is 1.8 times of the smooth SS surface. The increased site density of the nanoporous surface obtained by EPD enhanced greatly the nucleate pool boiling

An Improved Modeling for Low-grade Organic Rankine Cycle Coupled with Optimization Design of Radial-inflow Turbine

This document is the Accepted Manuscript of the following article: Lijing Zhai, Guoqiang Xu, Jie Wen, Yongkai Quan, Jian Fu, Hongwei Wu, and Tingting Li, ‘An improved modeling for low-grade organic Rankine cycle coupled with optimization design of radial-inflow turbine’, Energy Conversion and Management, Vol. 153: 60-70, December 2017. Under embargo. Embargo end date: 10 October 2018. The final, published version is available online at DOI: https://doi.org/10.1016/j.enconman.2017.09.063. Published by Elsevier Ltd.Organic Rankine cycle (ORC) has been proven to be an effective and promising technology to convert low-grade heat energy into power, attracting rapidly growing interest in recent years. As the key component of the ORC system, turbine significantly influences the overall cycle performance and its efficiency also varies with different working fluids as well as in different operating conditions. However, turbine efficiency is generally assumed to be constant in the conventional cycle design. Aiming at this issue, this paper couples the ORC system design with the radial-inflow turbine design to investigate the thermodynamic performance of the ORC system and the aerodynamic characteristics of radial-inflow turbine simultaneously. The constrained genetic algorithm (GA) is used to optimize the radial-inflow turbine with attention to six design parameters, including degree of reaction, velocity ratio, loading coefficient, flow coefficient, ratio of wheel diameter, and rotational speed. The influence of heat source outlet temperature on the performance of the radial-inflow turbine and the ORC system with constant mass flow rate of the heat source and constant heat source inlet temperature is investigated for four kinds of working fluids. The net electrical powers achieved are from few tens kWs to one hundred kWs. The results show that the turbine efficiency decreases with increasing heat source outlet temperature and that the decreasing rate of turbine efficiency becomes faster in the high temperature region. The optimized turbine efficiency varies from 88.06% (using pentane at the outlet temperature of 105 ºC) to 91.01% (using R245fa at the outlet temperature of 80 ºC), which appears much higher compared to common values reported in the literature. Furthermore, the cycle efficiency increases monotonously with the growth of the heat source outlet temperature, whereas the net power output has the opposite trend. R123 achieves the maximum cycle efficiency of 12.21% at the heat source outlet temperature of 110 ºC. Based on the optimized results, the recommended ranges of the key design parameters for ORC radial-inflow turbine are presented as well.Peer reviewe

Performance analysis of a new deep super-cooling two-stage organic Rankine cycle

This document is the Accepted Manuscript version of the following article: Y. Yuan, G. Xu, Y. Quan, H. Wu, G. Song, W. Gong, and X. Luo, ‘Performance analysis of a new deep super-cooling two-stage organic Rankine cycle’, Energy Conversion and Management, Vol. 148: 305-316, September 2017. The final, definitive version is available online at doi:https://doi.org/10.1016/j.enconman.2017.06.006. Published by Elsevier.In this article, a new deep super-cooling two-stage organic Rankine cycle (DTORC) is proposed and evaluated at high temperature waste heat recovery in order to increase the power output. A thermodynamic model of recuperative organic rankine cycle (ORC) is also established for the purpose of comparison. Furthermore, a new evaluation index, effective heat source utilization, is proposed to reflect the relationship among the heat source, power output and consumption of the waste heat carrier. A simulation model is formulated and analysed under a wide range of operating conditions with the heat resource temperature fixed at 300℃. Hexamethyldisiloxane (MM) and R245fa are used as the working fluid for DTORC, and MM for ORC. In the current work, the comparisons of heat source utilization, net thermal efficiency as well as the total surface area of the heat exchangers between DTORC and RC are discussed in detail. Results show that the DTORC performs better than ORC at high temperature waste heat recovery and it could increase the power output by 150%. Moreover, the maximum net thermal efficiency of DTORC can reach to 23.5% and increased by 30.5% compared with that using ORC, whereas the total surface areas of the heat exchangers are nearly the same.Peer reviewe

Multistage auto-ignition of undiluted methane/air mixtures under engine-relevant condition

Gas-phase auto-ignition delay times (IDTs) of methane/“air” (21% O2/79% Ar) mixtures were measured behind reflected shock waves, using a kinetic shock tube. Experiments were performed at fixed pressure of 1.8 MPa and equivalence ratios of 0.5 and 1.0, over the temperature range of 800–1000 K. Overall, the effect of equivalence ratio on IDT is negligible at entire temperatures measured in this study. The difference from traditional ignition regime at high temperatures, the undiluted methane/air mixtures present a four-stage ignition process at lower temperatures, namely deflagration delay, deflagration, deflagration-detonation transition, and detonation. Four popular kinetic mechanisms, UBC Mech 2.1, GRI Mech 3.0, Aramco Mech 2.0, and USC Mech 2.0, were used to simulate the new measurements. Only UBC Mech 2.1 showed satisfactory predictions in the reactivity of the undiluted methane mixtures; it was, thus, adopted to perform sensitivity analysis for identifying dominant reactions in the ignition process. The difference in channels contributing ȮH radicals causes a reduced global activation energy with decreasing temperatures.Keywords: Methane; multistage ignition; shock tube; sensitivity analysi

Contrastive study of flow and heat transfer characteristics in a helically coiled tube under uniform heating and one-side heating

One-side heated helically coiled tubes, which are generally applied in various industrial applications such as the water cooled wall in power plant boilers though, have not been thoroughly studied. To investigate the flow and heat transfer characteristics in this case, numerical simulation of the flow in a helically coiled tube is performed under uniform and non-uniform (heating on the inner coil side wall) heat flux boundary conditions for both laminar and turbulent flows. Temperature distributions, secondary flow distributions, average Nusselt number variation with respect to Reynolds number and local Nusselt number along the periphery on the wall in the fully developed section are discussed contrastively under the two different heating conditions. It is found that the secondary flow distributions are hardly affected by changing heating method, however, a larger temperature gradient can be found for one-side heating condition. The average Nusselt numbers are close for laminar flow under the two heating methods, but one-side heating shows 7–10% lower average Nusselt numbers than uniform heating for turbulent flow, thus a new correlation of average Nusselt number for turbulent flow and one-side heating is proposed. Furthermore, a special point on the inner wall where the local Nusselt numbers are almost the same when carrying out different heating conditions in laminar and turbulent flows is found, which should be useful for measuring unknown parameters

Uniform design for the optimization of Al2O3 nanofilms produced by electrophoretic deposition

Surface modification by means of nanostructures is of interest to enhance boiling heat transfer in various applications including the organic Rankine cycle (ORC). With the goal of obtaining rough and dense aluminum oxide (Al2O3) nanofilms, the optimal combination of process parameters for electrophoretic deposition (EPD) based on the uniform design (UD) method is explored in this paper. The detailed procedures for the EPD process and UD method are presented. Four main influencing conditions controlling the EPD process were identified as nanofluid concentration, deposition time, applied voltage and suspension pH. A series of tests were carried out based on the UD experimental design. A regression model and statistical analysis were applied to the results. Sensitivity analyses of the effect of the four main parameters on the roughness and deposited mass of Al2O3 films were also carried out. The results showed that Al2O3 nanofilms were deposited compactly and uniformly on the substrate. Within the range of the experiments, the preferred combination of process parameters was determined to be nanofluid concentration of 2 wt.%, deposition time of 15 min, applied voltage of 23 V and suspension pH of 3, yielding roughness and deposited mass of 520.9 nm and 161.6 × 10− 4 g/cm2, respectively. A verification experiment was carried out at these conditions and gave values of roughness and deposited mass within 8% error of the expected ones as determined from the UD approach. It is concluded that uniform design is useful for the optimization of electrophoretic deposition requiring only 7 tests compared to 49 using the orthogonal design method

Numerical analysis of the axial heat conduction with variable fluid properties in a forced laminar flow tube

This document is the Accepted Manuscript version of the following article: Lijing Zhai, et al, ‘Numerical analysis of the axial heat conduction with variable fluid properties in a forced laminar flow tube’, International Journal of Heat and Mass Transfer, Vol. 114: 238-251, November 2017. Under embargo until 22 June 2018. The final, definitive version is available online at doi: https://doi.org/10.1016/j.ijheatmasstransfer.2017.06.041.In this article, a theoretical model is developed to investigate the effects of the axial heat conduction on the laminar forced convection in a circular tube with uniform internal heat generation in the solid wall. In the current work, three different fluids, i.e. water, n-decane and air, are selected on purpose since their thermophysical properties show different behavior with temperature. The effects of the axial heat conduction with varying dynamic viscosity and/or varying thermal conductivity are investigated in a systematic manner. Results indicate that the variable-property effects could alleviate the reduction in Nusselt number (Nu) due to the axial heat conduction. For the case of Peclet number (Pe) equal to 100, wall thickness to inner diameter ratio of 1 and solid wall to fluid thermal conductivity ratio of 100, the maximum Nu deviation between constant and variable properties are up to 7.33% at the entrance part for water in the temperature range of 50℃, and 4.45% at the entrance part for n-decane in the temperature range of 120℃, as well as 2.20% at the ending part for air in the temperature range of 475℃, respectively. In addition, the average Nu deviation for water, n-decane and air are 3.24%, 1.94% and 1.74%, respectively. Besides, Nu decreases drastically with decreasing Pe when Pe≤500 and with increasing solid wall to fluid thermal conductivity ratio ( ) when ≤100. It is also found that variable properties have more obvious effects on the velocity profile at the upstream part while more obvious effects on the temperature profile at the downstream part.Peer reviewe

Effect of windage heating on a micro high-speed rotor-stator cavity

The main objective of this work is to investigate the effect of windage heating on the micro high-speed rotor-stator cavity. The influences of centrifugal force and spacing on the windage heating are analyzed with and without the change of gap ratio respectively. The results demonstrate that there is no difference in the flow structure between micro and large-scale rotor-stator cavities at the same rotational Reynolds number and gap ratio. However, the windage heating induced by the larger centrifugal force and smaller spacing brings the different heat transfer laws for the micro rotor-stator cavity. The larger centrifugal force weakens the local heat transfer near the rotor periphery. Such influence can be strengthened at higher rotational Reynolds numbers and lower rotor excess temperatures. Besides, the smaller spacing further enhances the windage heating and reduces the average heat transfer especially under the condition of lower gap ratio. The findings of this work contribute to the design of rotor-stator cavity utilized in the micro rotating machinery

Tunable Van Hove Singularity without Structural Instability in Kagome Metal CsTi3Bi5

In kagome metal CsV_{3}Sb_{5}, multiple intertwined orders are accompanied by both electronic and structural instabilities. These exotic orders have attracted much recent attention, but their origins remain elusive. The newly discovered CsTi_{3}Bi_{5} is a Ti-based kagome metal to parallel CsV_{3}Sb_{5}. Here, we report angle-resolved photoemission experiments and first-principles calculations on pristine and Cs-doped CsTi_{3}Bi_{5} samples. Our results reveal that the van Hove singularity (vHS) in CsTi_{3}Bi_{5} can be tuned in a large energy range without structural instability, different from that in CsV_{3}Sb_{5}. As such, CsTi_{3}Bi_{5} provides a complementary platform to disentangle and investigate the electronic instability with a tunable vHS in kagome metals