Numerical simulation of emulsified fuel spray combustion with puffing and micro-explosion

The purpose of this study was to develop numerical simulation of spray combustion of emulsified fuel with considering puffing and micro-explosion. First, a mathematical model for puffing was proposed. In the proposed puffing model, the rate of mass change of a droplet during puffing was expressed by the evaporation rate of dispersed water and the mass change rate due to fine droplets spouted from the droplet surface. The mass change rate due to fine droplets was related to the evaporation rate of the dispersed water and each liquid content. This model had only one experimental parameter. The essential feature of this model was that it was simple to apply to numerical simulation of spray combustion. First, the validity of the proposed puffing model was investigated with the experimental results for a single droplet. The calculated results for a single droplet with the experimental parameter varying from 5.0 to 10 were in good agreement with the experimental results. Moreover, numerical simulation of spray combustion of emulsified fuel was carried out. The occurrence of puffing and micro-explosion was determined by the inner droplet temperature. When micro-explosion occurred, a droplet changed to vapor rapidly. When the proposed puffing model was used in numerical simulation of spray combustion, the experimental parameter in the puffing model was determined for each droplet by random numbers within the range 5.0-10. The calculated results of spray combustion of emulsified fuel without considering puffing or micro-explosions were different from the experimental results even where combustion reactions were almost terminated. Meanwhile, the calculated results when considering puffing and micro-explosions were in good agreement with experimental results at the same location.     

Zero‐flux zone model analysis of a single bubble in boiling liquid

We start from a bubble, the basic unit of boiling, to explore liquid boiling. Research indicates that the heat and mass transfer between liquid and vapor is the determinant factor of boiling heat transfer. We have analyzed interfacial vaporization and condensation of a single bubble in boiling liquid based on a zero‐flux zone model. We have deduced the expression of zero‐flux angle and discussed the relationship between zero‐flux zone and the other parameters in order to comprehend the mechanism of boiling heat transfer. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 249–256, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10026     

Shape of “True” transition boiling curve and two‐mode boiling

Two‐mode boiling could occur on the same heating element, which provided information on the criterion separating the stable and metastable boiling regimes. The “equal‐area” criterion based on the complete boiling curve interpreted the criterion of coexistence of the two boiling modes. However, the transition boiling curve could not be constructed by the pool boiling tests. Literature works adopted various correlations of transition boiling curves for analysis, but failed to recognize the possible errors incorporated in such an approximation. This paper demonstrated the relative errors embedded in calculating the equal‐area criterion by assuming various shapes of the transition boiling curve. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(7): 593–601, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10113     

Fundamental study on biomass‐fuelled ceramic fuel cell

Recent development in the advanced intermediate temperature (400 to 700°C) ceramic fuel cell (CFC) research brings up feasibility and new opportunity to develop innovative biomass‐fuelled CFC technology. This work focuses on fundamentals of the biomass‐fuelled CFCs based on available biofuel resources through thermochemical conversion technologies. Both real producer gas from biomass gasification and imitative compounded gas were used as the fuel to operate the CFCs in the biomass CFC testing station. The composition of the fuel gas was varied in a wide range of practices of the present conversion technology both in KTH and Shandong Institute of Technology (SDIT). CFC performances were achieved between 100 and 700 mW cm^?2 at 600–800°C corresponding to various gas compositions. A high performance close to 400 mW cm^?2 was obtained at 600°C for the gas with the composition of H₂ (50 per cent)+CO (15 per cent)+CO₂ (15 per cent)+N₂ (20 per cent) and more than 600 mW cm^?2 for the H₂ (55 per cent)+CO (28 per cent)+CO₂ (17 per cent) at 700°C. This paper presents the experimental results and discusses the fundamentals and future potentiality on the biomass fuelled CFCs. Copyright © 2002 John Wiley & Sons, Ltd.     

A parametric study of burning of multicomponent droplets in a dilute spray

The model for sphericosymmetric thin‐flame combustion of a multicomponent fuel droplet in a dilute spray using a unit cell approach, developed in the companion paper, has been used for studying the interaction effect between droplets. The effects of droplet spacing, ambient oxidizer concentration, ambient temperature and pressure have been considered. Droplet life increases with decrease in droplet spacing, ambient temperature and ambient oxidizer concentration. However, droplet life has a weak dependence on ambient pressure. Copyright © 2001 John Wiley & Sons, Ltd.     

A study of thin‐flame quasisteady sphericosymmetric combustion of multicomponent fuel droplets. Part I: modelling for droplet surface regression and non‐unity gas‐phase Lewis number

A model for sphericosymmetric thin‐flame combustion of multicomponent fuel droplets has been developed in the first part of this two‐part work. The model incorporates effects of droplet surface regression and gas‐phase Lewis number. It is observed that both these effects affect the results substantially. The study also reveals the transient nature of the combustion process. Copyright © 1999 John Wiley and Sons, Ltd.     

Membrane‐less micro fuel cell system design and performance: An overview

Researchers interest in using fuel cells as a power source has grown because fuel cells are environmentally friendly. However, fuel cells still present challenges due to their performance and cost. This limits the commercialization of fuel cell systems, particularly in liquid fuel cells. One of the major obstacles is the Nafion membrane. The Nafion membrane is extremely expensive and causes the “fuel crossover phenomenon.” Therefore, researchers have proposed a membrane‐less fuel cell that eliminates the need of a membrane in the system mainly in micro fuel cells. Membrane‐less fuel cell has shown an improvement on power density by approximately 12% compared with conventional type of proton electrolyte membrane fuel cell. However, there still a lack of information on system design and performance. Therefore, the main objective of this review is to present an extensive study focusing on the geometrical system design and performance of a membrane‐less fuel cell system. It also presents the different types of membrane‐less fuel cell systems. Lastly, it highlights the current problems and potentials to improve the performance of the system. Finally, it is observed that the cost of a membrane fuel cell can be reduced by 20% to 40% compared with the conventional type of fuel cell.     

Numerical investigation of the effects of fuel spray type on the interaction of fuel spray and hot porous medium

The interaction between two types of fuel spray and a hot porous medium is studied numerically by using an improved version of KIVA-3V code. The improved KIVA-3V code is incorporated with an impingement model, a heat transfer model and a linearized instability sheet atomization (LISA) model to model the hollow cone spray. An evaporating fuel spray impingement on a hot plane surface was simulated under conditions of experiments performed by Senda to validate the reasonability of the KIVA-3V code. The numerical results conform well with experimental data for spray radius in the liquid and the vapor phases. Computational results on the interaction of two types of the fuel spray and the hot porous medium show that the fuel spray can be split, which provides conditions for quick evaporation of fuel droplets and mixing of fuel vapor with air. The possibility of fuel droplets from hollow cone spray crossing the porous medium reduces compared with that from solid cone spray, with the same initial kinetic energy of fuel droplets in both injection types. __________ Translated from Journal of Engineering Thermophysics, 2007, 28(2): 354–356 [译自: 工程热物理学报]     

Numerical study of serpentine flow‐field cooling plates on PEM fuel cells performance

The effect of a cooling plate on a PEM fuel cell was studied by three‐dimensional CFD modeling. The cyclic cell and the single cell were compared for the evaluation of the influence of cooling plate. The cyclic cell consisted of a single cell and a two‐channel serpentine flow‐field coolant, which then repeats by using a cyclic boundary on both ends. The single cell was composed of an active area of 200 cm² and a 10‐channel serpentine flow field. The following sets of equations were used in the model: the conservation of electrical current, the mass conservation of gases species, the Navier–Stokes equation, the energy balance, and the water phase change model. Comparison of cyclic cell and single cell shows that the voltage of cyclic cell was reduced at high current densities because of the increased ohmic losses. This was caused by the combined effect of membrane dehydration and higher local temperature. However, the cyclic cell showed more uniform current density distribution than the single cell, and this is attributed to the use of cooling plate. Increasing the coolant flux enhanced the cell performance by reducing the ohmic loss. Copyright © 2011 John Wiley & Sons, Ltd.     

An experimental study on spray auto-ignition of RP-3 jet fuel and its surrogates

Jet fuel is widely used in air transportation, and sometimes for special vehicles in ground transportation. In the latter case, fuel spray auto-ignition behavior is an important index for engine operation reliability. Surrogate fuel is usually used for fundamental combustion study due to the complex composition of practical fuels. As for jet fuels, two-component or three-component surrogate is usually selected to emulate practical fuels. The spray auto-ignition characteristics of RP-3 jet fuel and its three surrogates, the 70% mol n-decane/30% mol 1,2,4-trimethylbenzene blend (Surrogate 1), the 51% mol n-decane/49% mol 1, 2, 4-trimethylbenzene blend (Surrogate 2), and the 49.8% mol n-dodecane/21.6% mol iso-cetane/28.6% mol toluene blend (Surrogate 3) were studied in a heated constant volume combustion chamber. Surrogate 1 and Surrogate 2 possess the same components, but their blending percentages are different, as the two surrogates were designed to capture the H/C ratio (Surrogate 1) and DCN (Surrogate 2) of RP-3 jet fuel, respectively. Surrogate 3 could emulate more physiochemical properties of RP-3 jet fuel, including molecular weight, H/C ratio and DCN. Experimental results indicate that Surrogate 1 overestimates the auto-ignition propensity of RP-3 jet fuel, whereas Surrogates 2 and 3 show quite similar auto-ignition propensity with RP-3 jet fuel. Therefore, to capture the spray auto-ignition behaviors, DCN is the most important parameter to match when designing the surrogate formulation. However, as the ambient temperature changes, the surrogates matching DCN may still show some differences from the RP-3 jet fuel, e.g., the first-stage heat release influenced by low-temperature chemistry.     

Prediction of critical heat flux on natural convective boiling in vertical short‐thick tube submerged in saturated liquid

An experimental and semitheoretical study was carried out for the critical heat flux (CHF) on natural convective boiling in uniformly heated vertical short‐thick tubes and vertical short‐thick annular tubes submerged in saturated liquids. By adapting a mathematical dealing method based on the theoretical formulas of CHF of both the natural convective boiling in vertical narrow‐long tubes and the pool boiling, a simple semitheoretical formula was derived. The new formula expands the prediction range of CHF from pool boiling of vertical plates to very long vertical tubes and agrees well with the data of the tubes, annular tubes submerged in water or other liquids under various pressure conditions. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 402–410, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10103     

Behaviors of micro‐layer in micro‐channel boiling system applying laser extinction method

To elucidate the mechanism and characteristics of boiling heat transfer in a micro‐channel vaporizer, the experimental investigation of the micro‐layer thickness that formed between the heating surface and vapor generated was important. The micro‐layer thickness was measured applying the laser extinction method for channel gap sizes of 0.5, 0.3, and 0.15 mm. It was clarified that the gap size, the rate of bubble growth, and the distance from the incipient bubble site have an effect on the micro‐layer thickness in a micro‐channel boiling system. The initial micro‐layer thickness grew with an increase of the velocity of bubble forefront to moderate the value of the velocity. In the region of greater velocity, the thickness was constant for each gap. The distributions of the initial thickness of micro‐layer on the heat transfer surface were shown. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 35–46, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20096     

Numerical study of hydrogen‐enriched methane–air combustion under ultra‐lean conditions

The demand for gas turbines that accept a variety of fuels has continuously increased over the last decade. Understanding the effects of varying fuel compositions on combustion characteristics and emissions is critical to designing fuel‐flexible combustors. In this study, the combustion characteristics and emissions of methane and hydrogen‐enriched methane were both experimentally and numerically investigated under ultra‐lean conditions (Ø ≤ 0.5). This study was performed using global mechanisms with a one‐step mechanism by Westbrook and Dryer and a two‐step mechanism with an irreversible and reversible CO/CO₂ step (2sCM1 and 2sCM2). Results show that the 2sCM2 mechanism under‐predicted the temperature, major species, and NO_x by more than 100% under ultra‐lean conditions; thus, we proposed a modified‐2sCM2 mechanism to better simulate the combustion characteristics. The mechanisms of Westbrook, 2sCM1, and modified 2sCM2 predicted the temperature and the CO₂ emission with an average deviation of about 5% from the experimental values. Westbrook and 2sCM1, however, over‐predicted the NO_x emission by approximately 81% and 152%, respectively, as compared with an average under‐prediction of 11% by the modified‐2sCM2 mechanism. The numerical results using the proposed modified‐2sCM2 mechanism shows that the presence of hydrogen in the fuel mixture inhibits the oxidation of methane that led to the formation of unburned hydrocarbons in the flame. We also showed that for any given fuel compositions of H₂/CH₄, there is an optimum equivalence ratio at which the pollutant emissions (CO and NO_x) from the combustor are minimal. Zero CO and 5 ppm NO_x emissions were observed at the optimal equivalence ratio of 0.45 for a fuel mixture containing 30% H₂. The present study provides a basis for ultra‐lean combustion toward achieving zero emissions from a fuel‐flexible combustor. Copyright © 2016 John Wiley & Sons, Ltd.     

Neutron physics of the liquid‐fuel heat‐pipe reactor concept with molten salt fuel—Static calculations

A liquid‐fuel heat‐pipe reactor (LFHPR) is a novel fast heterogeneous reactor developed by Harbin Engineering University, China, on the basis of liquid‐fuel reactor designs and the heat‐pipe reactor concept. In the concept, the reactor abandons the graphite moderator and keeps neither fuel tubes arranged in the graphite nor fuel rings around the heat pipe. Instead, the reactor applies molten salt fuels, molten metallic eutectic fuels, or other fuels in liquid form. The heat generated in the reactor is removed by the heat pipes driven by liquid metals. With this change, an LFHPR is much more flexible in design and application and able to achieve several advanced features compared with conventional heat‐pipe reactors. In this paper, we describe the general reactor design of an LFHPR, discuss its potential advantages, and give a preliminary verification of the neutron physical feasibility for the reference case, which uses molten salt as the fuel, by using both Monte Carlo and deterministic methods. Results show that the LFHPR yields a hard neutron spectrum that brings a very good neutron economy and is a promising application for breeding. From our approach, we conclude that the proposed LFHPR has a very high power density and high negative temperature feedback coefficient.     

Two‐phase flow boiling characteristics of alternatives to R‐22 inside air/refrigerant enhanced surface tubing

In this paper, an experimental study on the heat transfer characteristics of two‐phase flow boiling of alternative zeotropic refrigerant mixtures to R‐22, on air/refrigerant horizontal enhanced surface tubing is presented. The new alternatives considered in this study are: R‐507, R‐404A, R‐408A, R‐407C, and R‐410A. The experimental data showed that R‐22 has the highest heat transfer rate compared to the other blends in the range investigated. Furthermore, it was also quite evident from these data that R‐410A has the highest pressure among the blends under investigation for Reynolds number greater than 3.5×10⁴. However, for Reynolds number less than 3.6×10⁴, it appears from the data that R‐22 has the highest pressure drop compared to other refrigerants under investigation. Copyright © 2000 John Wiley & Sons, Ltd.     

Non‐isothermal multi‐phase modeling of PEM fuel cell cathode

In this study, numerical simulation has been carried out for the heat transfer and temperature distribution in the cathode of polymer electrolyte membrane fuel cells along with the multi‐phase and multi‐species transport under the steady‐state condition. The commercial software, COMSOL Multiphysics, is used to solve the conservation equations for momentum, mass, species, charge and energy numerically. The conservation equations are applied to the solid, liquid and vapor phases in the bipolar plate and gas diffusion (GDL) and catalyst layers of a two‐dimensional cross section of the cathode. The catalyst layer is assumed to be a finite domain and the water production in the catalyst layer is considered to be in the liquid form. The temperature distribution in the cathode is simulated and then the effects of the relative humidity of the air stream, the permeability of the cathode and the flow channel shoulder to channel width ratio are investigated. It is shown that the highest temperature change, both in the in‐plane and across‐the‐plane directions, occurs in the GDL, while the highest temperature is reached in the catalyst layer. The distribution of temperature in the bipolar plate is shown to be relatively uniform due to the high thermal conductivity of the plate. A decrease in the inlet relative humidity of the air stream results in the decrease of the maximum temperature due to the absorption of heat during the evaporation of liquid water in the GDL and catalyst layer. The non‐uniformity of the temperature distribution, especially in the catalyst layer, is observed with the increase of the permeability of the cathode. Similarly, the decrease of the channel shoulder to channel width ratio leads to a non‐uniform distribution of temperature especially under the channel areas. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental study of pool boiling heat transfer of water‐based magnetic fluid on a horizontal heater

The effect of mass concentration of magnetic particles and an applied magnetic field on pool boiling heat transfer of water‐based magnetic fluid on a horizontal heater was investigated. The experimental results show that high‐concentration magnetic fluid deteriorates boiling heat transfer, while middle‐ and low‐concentration magnetic fluid enhances the boiling heat transfer. There was an optimum concentration in which the enhancement of boiling heat transfer was the best. Conclusions were the same with an applied magnetic field that enhances the boiling heat transfer of magnetic fluid further. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(3): 180–187, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20054     

Heat transfer prediction of air‐to‐refrigerant two‐phase flow boiling of alternatives to HCFC‐22 inside air/refrigerant enhanced surface tubing

In this paper, an experimental study on the heat transfer characteristics of two‐phase flow boiling of some alternative refrigerants to HCFC‐22, on air/refrigerant horizontal enhanced surface tubing, is presented. Correlations have been proposed to predict the heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops of alternatives to R‐22; such as R‐507, R‐404A, R‐407C, R‐410A and R‐408A in two‐phase flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture's composition. It was found that the correlations were applicable to the entire heat and mass flux, investigated in the present study, for the proposed blends under question. The deviation between the experimental and predicted values for the heat transfer coefficient and pressure drop were less than ±20, and ±35 per cent, respectively, for the majority of data. Copyright © 2000 John Wiley & Sons, Ltd.     

Power flattening study of ultra‐long cycle fast reactor using thorium fuel

This paper presents a design study of power shape flattening for an optimized ultra‐long cycle fast reactor with a power rate of 1000 MWe in order to mitigate the power peaking issue and improve the safety with a lower maximum neutron flux and reactivity swing. There are variations in the core designs by loading thorium fuel or zoning fuels in the blanket region and the bottom driver region of ultra‐long cycle fast reactor with a power rate of 1000 MWe. While it has lower breeding performance in a fast breeder reactor, thorium fuel is one of the promising fuel options for future reactors because of its abundance and its safety characteristics. It has been confirmed that the thorium fuels, when loaded into the center region of a reactor core, lower the power peaking factor from 1.64 to 1.25 after 20 years and achieves a more flattened radial power distribution. This consequently reduces the maximum neutron flux and the speed of the active core moving from 3.0 cm/year to 2.5 cm/year on the average over the 60‐year reactor operation. It has been successfully demonstrated that the three‐zone core is the most optimized core, has the most flattened radial power shape, and is without any compromise in the nature of long cycle core, from the neutronics point of view, in terms of average discharge burnup and breeding ratio. Copyright © 2016 John Wiley & Sons, Ltd.     

A study on boiling in a horizontal channel with a rectangular section (in the case of a heated bottom wall)

The characteristics of boiling in a horizontal channel with changing conditions of the length of the heated wall and the channel height have been studied experimentally. Behavior of bubbles on the heated wall and growth of bubbles in the channel were observed by a high‐speed camera. As a result, the behavior of the growth of bubbles which was classified into three types according to channel height had an influence on the time variation of the degree of superheat, heat transfer, and burnout heat flux in the channel. When the liquid on the bottom wall became thin, nucleate boiling with a vapor dome was observed on the heated wall. © 2000 Scripta Technica, Heat Trans Asian Res, 29(6): 459–472, 2000