Inlet Temperature Tin Research Articles

Numerical analysis of NOx formation mechanisms and emission characteristics with different types of reactants dilution during MILD combustion of methane and coke oven gas

A numerical study is conducted to emulate the MILD combustion conditions in actual industrial furnaces and to explore the effects of different types of reactants dilution on NOx formation mechanisms and emission characteristics of methane and coke oven gas (COG) combustion. A well-stirred reactor (WSR) is utilized to generate high-temperature flue gas that dilutes and preheats the fuel and the oxidizer at different dilution ratios. The model with two plug-flow reactors and one opposed-flow diffusion flame reactor is selected for the simulations. The OH radical profile analysis indicates that COG combustion maintains a larger reaction zone due to high hydrogen concentration as compared to methane combustion. The results show that there exists a critical oxygen percentage that divides MILD combustion into two different combustion regimes based on the inlet temperature Tin of the reactants. And the critical oxygen percentage to achieve MILD combustion is about 9.5% and 7.5% for methane and COG, respectively. Moreover, the effect of different dilution types on the NOx emissions indicates that the NOx emission index (EINO) value for the fuel dilution case is about 0.2 g-NO/kg-fuel whereas the value for the oxidizer dilution case is around 0.4 g-NO/kg-fuel at the dilution ratio of 0.9. The comparison between the EINO values for fuel dilution and oxidizer dilution cases suggests that the fuel dilution is more effective to reduce NOx emission than the oxidizer dilution. The relative contribution analysis of different NOx formation routes reveals that the dominant NOx formation route is the prompt route for methane combustion whereas the N2O intermediate and NNH routes are also important for COG combustion. The results provide a good insight into the NOx formation mechanism and assist in the design of a low NOx burner of MILD combustion.

Numerical investigations on solid-fueled ramjet inlet thermodynamic properties effects on generating self-sustained combustion instability

In this work, combustion instability of the solid-fueled ramjet (involving diffusion-dominant combustion process) is investigated using a numerical model. Effects of inlet thermodynamic properties, such as inlet temperature Tin, Mach number Ma and mass flow rate m˙a on generating combustion instability are evaluated. The 2-dimensional (2D) model is first validated with 3-dimensional (3D) model and the experimental data. Further validations are conducted on chemical reaction model, heat transfer model and diffusion models and comparing the acoustic signature of a Ramjet combustor and mode-shapes with the experimental data available. It is then used to identify stable and unstable acoustic modes exited by unsteady heat release in the combustor. Large-scale temperature waves are observed to be convected downstream. To clarify the mechanism of such limit cycles, Rayleigh criterion is utilized by analyzing the phase difference between the acoustic pressure and unsteady heat release rate. In addition, attempts are made to determine Rayleigh index and the oscillations' growth rate. Finally, it is found that the inlet temperature plays little roles on affecting the limit cycle intensity. This is quite different from m˙a and Ma. Larger m˙a and/or Ma, more intensified limit cycle oscillations. In general, the model provides a useful low-computational-cost numerical platform to investigate thermoacoustic instability in a Ramjet combustor.

NOx emission and thermal performances studies on premixed ammonia-oxygen combustion in a CO2-free micro-planar combustor

Unlike hydrocarbon fuel, ammonia is a carbon-free and renewable energy source. It is also regarded as one of the potential energy carriers. However, ammonia combustion for power generation is not well studied under micro-scale conditions, especially concerning nitrogen oxides (NOx) emission. For this, thermal performances and NO emission characteristics of premixed ammonia/oxygen combustion are numerically investigated on a micro-planar combustor. The effects of 1) the equivalence ratio ϕ, 2) inlet temperature Tin and 3) inlet pressure Pin are examined. The outer wall mean temperature (OWMT) is found to vary non-monotonically, as the mixture varies from lean to rich conditions, with the peak occurring at ϕ = 0.9 mainly due to the optimal heat transfer performance. However, a low ϕ could lead to the high nitric oxides (NO) concentrations because of the high flame temperature as well as O atom concentrations. Up to 75.5% of NO reduction could be achieved, as ϕ is optimized. Furthermore, increasing Tin is shown to be associated with a low OWMT and NO concentration. In addition, varying Pin is shown to lead to not only OWMT being changed but also NO formation being mitigated. The decrease in NO concentration for a high Pin is mostly attributable to the short residence time of a high flame temperature in the channel and low OH concentrations. This work reveals that optimizing the operating thermodynamic parameters is an effective means to reduce NO emissions and to improve thermal performances.

Experimental study on the flow boiling oscillation characteristics in a rectangular multiple micro-channel

Multiple micro-channels can be used as heat sink components of many electronic devices due to high heat transfer capacity. In this paper, the flow boiling oscillation characteristics of a multiple micro-channel test section made of six rectangular micro-channels with an equivalent hydraulic diameter of 526.2 μm were experimentally studied. The experiments were operated with heat flux qeff 11.58–99.56 kW/m2, mass flux G 20.40–107.04 kg/m2·s and inlet temperature Tin = 55 °C. The flow pattern evolution process in the multiple micro-channel was visually studied and analyzed. Also, the pressure drop characteristics of the multiple micro-channel test section and two individual channels of the multiple micro-channel were measured and studied with time domain and frequency domain analyses, and the relationship between amplitude-frequency characteristics of test section pressure drop and qeff/G under typical working conditions was gained. The results showed that under oscillation conditions different flow pattern evolution modes may occur for different micro-channels due to the coupling effects of inertial force, the evaporation momentum, heating wall etc. During boiling, reverse flow of bubbles could be observed, and liquid may also be pushed upstream by the bubbles. The differences of oscillation waveform, frequency, phase position as well as the interaction effects of individual channels were found to get smaller with an increase of outlet equilibrium quality. Based on the amplitude-frequency characteristics, five flow boiling patterns were identified as boiling developed, i.e. the single phase pattern, the boiling onset pattern, the reverse flow onset pattern, the reverse flow infecting pattern, and the complete reverse flow pattern. The boundaries of the flow boiling patterns were very clear from a view of oscillation amplitude-frequency characteristics, and it was found that the boundaries can be roughly determined with parameter qeff/G.

Effect of Longitudinal Vortex Generator Location on Thermoelectric-Hydraulic Performance of a Single-Stage Integrated Thermoelectric Power Generator

Vortex generators have been widely used to enhance heat transfer in various heat exchangers. Out of the two types of vortex generators, transverse vortex generators and longitudinal vortex generators (LVGs), LVGs have been found to show better heat transfer performance. Past studies have shown that the implementation of these LVGs can be used to improve heat transfer in thermoelectric generator systems. Here, a built in module in COMSOL Multiphysics® was used to study the influence of the location of LVGs in the channel on the comprehensive performance of an integrated thermoelectric device (TED). The physical model under consideration consists of a copper interconnector sandwiched between p-type and n-type semiconductors and a flow channel for hot fluid in the center of the interconnector. Four pairs of LVGs are mounted symmetrically on the top and bottom surfaces of the flow channel. Thus, using numerical methods, the thermo-electric-hydraulic performance of the integrated TED with a single module is examined. By fixing the material size D, the fluid inlet temperature Tin, and attack angle β, the effects of the location of LVGs and Reynolds number were investigated on the heat transfer performance, power output, pressure drop, and thermal conversion efficiency. The location of LVGs did not have significant effect on the performance of TEGs in the given model. However, the performance parameters show a considerable change with Reynold's number and best performance is obtained at Reynold number of Re = 500.

An experimental study of two-phase pressure drop of acetone in triangular silicon micro-channels

Two-phase pressure drop was measured across a silicon micro-channels heat sink. Ten triangular micro-channels were fabricated in a silicon substrate by the etching technique in a silicon substrate. Each micro-channel is 300 μm in width and 212 μm in depth, forming the hydraulic diameter of 155.4 μm. Experimental studies were performed with acetone as working fluid under the following conditions: inlet pressure of Pin = 1.3–1.6 bar, inlet temperature Tin = 23.0–39.1 °C, mass velocity of G = 65.52–289.61 kg/m2 s, outlet quality of xe,out = 0.38–superheat, and heat flux of q = 141.92–481.08 kW/m2. The frictional pressure drop during flow boiling is estimated using two models, namely the homogeneous model and the separated flow model. In addition, the measured pressure drops are compared with those from a few correlation models available for macro-scales and mini/micro-scales. A new correlation is developed based on the Chisholm constant B as a function of mass flux. It is found that the new correlation can be used to predict the present experimental data within a mean absolute error (MAE) of 12.56%.

Combustion of CH4/O2/N2 in a well stirred reactor

Detailed kinetic calculations of chemical reactions using GRI-Mech 3.0 are carried out to characterize the combustion of CH4/O2/N2 mixture in a well-stirred reactor (WSR). Very wide ranges of the inlet temperature Tin (300 K–2700 K), nitrogen dilution concentration (0%–99%) and global equivalence ratio (0.5–4.0) are considered. The extinction and self-ignition temperatures (Tex and Tsi) of the CH4/O2/N2 mixture are identified by the ignition-extinction S-curve. Based on Tex, TsiTWSR, and the operative conditions (Tin and the mixture composition), the WSR combustion of CH4/O2/N2 can be quantitatively classified into several particular regimes. Moreover, the product composition and elementary chemical pathways of the CH4 oxidation are also examined. Results demonstrate that the WSR working temperature determines the chemical characteristics of the CH4 oxidation rather than the combustion regime.

High-pressure homogenisation of raw bovine milk. Effects on fat globule size distribution and microbial inactivation

Whole raw milk was processed using a 15 L h−1 homogeniser with a high-pressure (HP) valve immediately followed by a cooling heat exchanger. The influence of homogenisation pressure (100–300 MPa) and milk inlet temperature Tin (4°C, 14°C or 24°C) on milk temperature T2 at the HP valve outlet, on fat globule size distribution and on the reduction of the endogenous flora were investigated. The Tin values of 4–24°C led to milk temperatures of 14–33°C before the HP valve, mainly because of compression heating. High Tin and/or homogenisation pressure decreased the fat globule size. At 200 MPa, the d4.3 diameter of fat globules decreased from 3.8±0.2 (control milk) to 0.80±0.08 μm, 0.65±0.10 or 0.37±0.07 μm at Tin=4, 14°C or 24°C, respectively. A second homogenisation pass at 200 MPa (Tin=4°C, 14°C or 24°C) further decreased d4.3 diameters to about 0.2 μm and narrowed the size distribution. At all Tin tested, an homogenisation pressure of 300 MPa induced clusters of fat globules, easily dissociated with SDS, and probably formed by sharing protein constituents adsorbed at the fat globule surface. The total endogenous flora of raw milk was reduced by more than 1 log cycle, provided homogenisation pressure was ⩾200 MPa at Tin=24°C (T2∼60°C), 250 MPa at Tin=14°C (T2∼62°C), or 300 MPa at Tin=4°C (T2∼65°C). At all Tin tested, a second pass through the HP valve (200 MPa) doubled the inactivation ratio of the total flora. Microbial patterns of raw milk were also affected; Gram-negative bacteria were less resistant than Gram-positive bacteria.

Analysis and Reduction of the CH4-Air Mechanism at Lean Conditions

A palette of computational techniques are employed for the analysis and reduction of a detailed 53-species methane-air mechanism including nitrogen chemistry in a perfectly stirred reactor (PSR). The analysis of the mechanism for a PSR operating at an equivalence ratio φ = 0.5, inlet temperature Tin = 400°C and pressure p = 1 bar, enables a first reduction to a 26-species skeletal mechanism. Computational singular perturbation is then applied to find the species at quasi-steady state. The resulting 11-species reduced mechanism is tested at different operating conditions in the PSR, as well as in a reactor network of a PSR followed by a plug flow reactor (PSR), showing good aggreement with the full mechanism over a range of equivalence ratios up to stoichiometric.

Effect of Kinetic, Design, and Operating Parameters on Reactor Gain

Many industrial reactions are carried out in adiabatic tubular reactors. The desired inlet feed temperature is usually achieved by using the hot reactor effluent to preheat the cold feed in a feed−effluent heat exchanger (FEHE). This positive feedback of energy introduces the potential for open loop instability. Previous papers have explored the control of this type of process. The key parameter in FEHE/reactor systems is the reactor gain, i.e. the change in the reactor exit temperature Tout for a given change in the reactor inlet temperature Tin: KR = ΔTout/ΔTin. The larger the reactor gain, the more likely open loop instability will occur and the more difficult the control problem will be. This paper explores the impact of various kinetic, design, and operating parameters on reactor gain. Three reaction systems are studied. The first is the hypothetical reaction A + B → C, where kinetic parameters (activation energy and heat of reaction) can be varied. Then two real industrial reactions are studied: the hydrodealkylation of toluene (HDA process) and the chlorination of propylene (allyl chloride process), where design parameters (inlet reactant feed concentration and reactor length) are varied. Larger reactor gains occur for increases in reactant feed concentration, activation energy, and heat of reaction. Reactor gains can vary non-monotonically for some design parameters when interactions between concentration and temperature effects occur.

Tuning Proportional−Integral Controllers for Processes with Both Inverse Response and Deadtime

Many tuning methods have been proposed over the last half-century. Most apply to deadtime/lag processes, but several have studied either integrating processes or inverse response processes. Very few have explored processes in which both deadtime and inverse responses occur. This type of response is observed in adiabatic tubular reactors when reactor outlet temperature Tout is the controlled variable and reactor inlet temperature Tin is the manipulated variable. Typically the control structure uses a cascade arrangement in which the secondary loop uses furnace heat input or heat-exchanger bypassing to control Tin. This paper addresses the problem of controlling processes that exhibit both inverse response and deadtime. The Ziegler−Nichols tuning method recommended in the literature is shown to give poor performance at both small and large values of deadtime and positive-zero time constant. A new tuning method is proposed in which the PI tuning constants are presented as functions of the positive zero τz and the deadtime D. Results are given in the form of easy-to-use tuning charts.