Laiqin Luo

An Experimental Study of Shell-and-Tube Heat Exchangers With Continuous Helical Baffles

Two shell-and-tube heat exchangers (STHXs) using continuous helical baffles instead of segmental baffles used in conventional STHXs were proposed, designed, and tested in this study. The two proposed STHXs have the same tube bundle but different shell configurations. The flow pattern in the shell side of the heat exchanger with continuous helical baffles was forced to be rotational and helical due to the geometry of the continuous helical baffles, which results in a significant increase in heat transfer coefficient per unit pressure drop in the heat exchanger. Properly designed continuous helical baffles can reduce fouling in the shell side and prevent the flow-induced vibration as well. The performance of the proposed STHXs was studied experimentally in this work. The heat transfer coefficient and pressure drop in the new STHXs were compared with those in the STHX with segmental baffles. The results indicate that the use of continuous helical baffles results in nearly 10% increase in heat transfer coefficient compared with that of conventional segmental baffles for the same shell-side pressure drop. Based on the experimental data, the nondimensional correlations for heat transfer coefficient and pressure drop were developed for the proposed continuous helical baffle heat exchangers with different shell configurations, which might be useful for industrial applications and further study of continuous helical baffle heat exchangers. This paper also presents a simple and feasible method to fabricate continuous helical baffles used for STHXs.

Numerical investigation of heat transfer and fluid flow characteristics inside a wavy channel

The periodically fully developed laminar heat transfer and fluid flow characteristics inside a two-dimensional wavy channel in a compact heat exchanger have been numerically investigated. Calculations were performed for Prandtl number 0.7, and Reynolds number ranging from 100 to 1,100 on non-orthogonal non-staggered grid systems, based on SIMPLER algorithm in the curvilinear body-fitted coordinates. Effects of wavy heights, lengths, wavy pitches and channel widths on fluid flow and heat transfer were studied. The results show that overall Nusselt numbers and friction factors increase with the increase of Reynolds numbers. According to the local Nusselt number distribution along channel wall, the heat transfer may be greatly enhanced due to the wavy characteristics. In the geometries parameters considered, friction factors and overall Nusselt number always increase with the increase of wavy heights or channel widths, and with the decrease of wavy lengths or wavy pitches. Especially the overall Nusselt number significantly increase with the increase of wavy heights or channel widths, where the flow may become into transition regime with a penalty of strongly increasing in pressure drop.

Numerical simulation and optimization on heat transfer and fluid flow in cooling channel of liquid rocket engine thrust chamber

Purpose – To find the optimal number of channels of rocket engine thrust chamber, it was found that the optimal channel number is 335, at which the cooling effect of the thrust chamber cooling channel reaches the best, which can be helpful to design rocket engine thrust chamber.Design/methodology/approach – The commercial computational fluid dynamics (CFD) software FLUENT with standard k‐e turbulent model was used. The CFD method was validated via comparing with the available experimental data.Findings – It was found that both the highest temperature and the maximal heat flux through the wall on the hot‐gas side occurs about the throat region at the symmetrical center of the cooling channel. Owing to the strong curvature of the cooling channel geometry, the secondary flow reached its strongest level around the throat region. The typical values of pressure drop and temperature difference between the inlet and exit of cooling channel were 2.7 MPa and 67.38 K (standard case), respectively. Besides an optimal...

DSMC simulation of low-speed gas flow and heat transfer in 2D rectangular micro-channel

Effective boundary condition is one of the most critical problems in the computation of micro-channel flows with direct simulation Monte-Carlo (DSMC) method. In the present work, the implementation of DSMC with specified pressure boundary condition (PBC) was discussed in detail. The variations of gaseous local pressure, temperature and number density of the particles caused by the temperature difference between channel walls and gas were presented. It was found that with the increase of both gaseous compressibility and rarefaction, the pressure distribution along micro-channel became more nonlinear. Heat transfer occurred almost only at channel inlet and outlet, and the average wall heat flux increased almost linearly to the inlet-to-outlet pressure ratio. The computational results also showed that PBC was more suitable for the simulation of micro-channel flow problems than the conventional velocity boundary condition (VBC).

Genetic Algorithm Optimization for Primary Surfaces Recuperator of Microturbine

Recent years, Genetic Algorithm (GA) technique has been gotten much more attention in solving real-world problems, more successful genetic algorithms applications to engineering optimization have shown the technique has strong ability of global searching and optimizing based on various objectives for their optimal parameters. The technique may be applied to more complicated heat exchangers and is particularly useful for new types. It is important to optimize heat exchanger, for minimum volume/weight to save fabrication cost or for improved effectiveness to save energy consumption, under requirement of allowable pressure drop; simultaneously it is mandatory to optimize geometry parameters of heat plate from the technical and economic standpoints. In this paper, GA is used to optimize the Cross Wavy Primary Surface (CWPS) and Cross Corrugated Primary Surface (CCPS) geometry characteristic of recuperator in 100kW microturbine, in order to get more compactness and minimum volume and weight. Two kinds of fitness assignment methods were considered. Furthermore, the GA parameters were set optimally to yield smoother and faster fitness convergence. The comparison shows the superiority of GA and confirms its potential to solve the objective problem. When the rectangular recuperator core size and corrugated geometries are evaluated, in the CWPS the weight of recuperator decreases 12% or more, the coefficient of compactness increases by up to 19%, with an increase of total pressure drop by a percentage of 0.84 compared to the original design data, the total pressure drop as a percentage of the operating pressure is controlled to be less than 3%. In the CCPS area compactness is increased to 70% the initial design result by decreasing pitch and relatively high height of the passage, the weight decreases by 17% to 36%, depending on different inclination angle (θ). Comparatively the CCPS shows superior performance for use in compact recuperators of the future. The GA technique chooses from a variety of geometry characters, optimizes them and picks out the one which provides the closest fit to the recupertor for microturbine.Copyright

Numerical Simulation of Gas Flow and Heat Transfer in Cross-Wavy Primary Surface Channel for Microtubine Recuperators

Three-dimensional numerical simulation was conducted to investigate the flow field and heat transfer performance of the Cross-Wavy Primary Surface (CWPS) recuperators for microturbines. Using high-effective compact recuperators to achieve high thermal efficiency is one of the key techniques in the development of microturbine in recent years. Recuperators need to have minimum volume and weight, high reliability and durability. Most important of all, they need to have high thermal-effectiveness and low pressure-losses so that the gas turbine system can achieve high thermal performances. These requirements have attracted some research efforts in designing and implementing low-cost and compact recuperators for gas turbine engines recently. One of the promising techniques to achieve this goal is the so-called primary surface channels with small hydraulic dimensions. In this paper, we conducted a three-dimensional numerical study of flow and heat transfer for the Cross-Wavy Primary Surface (CWPS) channels with two different geometries. In the CWPS configurations the secondary flow is created by means of curved and interrupted surfaces, which may disturb the thermal boundary layers and thus improve the thermal performances of the channels. To facilitate comparison, we chose the identical hydraulic diameters for the above four CWPS channels. Since our experiments on real recuperators showed that the Reynolds number ranges from 150 to 500 under the operating conditions, we implemented all the simulations under laminar flow situations. By analyzing the correlations of Nusselt numbers and friction factors vs. Reynolds numbers of the four CWPS channels, we found that the CWPS channels have superior and comprehensive thermal performance with high compactness, i.e., high heat transfer area to volume ratio, indicating excellent commercialized application in the compact recuperators.Copyright

Experimental Investigation on Heat Transfer and Pressure Drop in a Microtubine Recuperator With Cross-Wavy Primary Surface Channels

Compact heat exchangers are used in a wide variety of applications. Typical utilization is a low-cost recuperator for power generation microturbines. In this scenario, a recuperator takes heat from the exhaust gas and preheats the compressor discharge air before it reaches the combustion chamber. To achieve thermal efficiency over 30%, recuperators with high thermal performance surfaces geometries are needed. It has been shown that Cross-Wavy Primary Surface (CWPS) has superior performance and high commercial potential in compact recuperators based on previous studies. In the present study, we successfully implemented a prototype recuperator with CWPS channels for a 100kW microturbine. The material we used in the recuperator core is a 0.12mm-thick stainless steel strip, which has good high-temperature mechanical and corrosion properties. The working mediums are compressed air and hot gas for the two sides of the recuperator. We tested comprehensively the thermal performance of the recuperator in terms of the overall heat transfer coefficients and friction factors vs. Reynolds numbers in the CWPS channels, with Reynolds number ranging from 250 to 400. The exhaust hot gas temperature was much non-uniform, indicating the importance of flow arrangement when designing the recuperator. We also investigated the heat transfer coefficients and friction factors vs. Reynolds numbers, and obtained corresponding correlations.Copyright

CFD Optimization of Gas Inlet Configuration for 100kW Microturbine Recuperator

Compact and efficient recuperator is an important component of a microturbine system. To ascertain the optimum gas cavity configuration of the recuperator in a 100kW-microturbine system, a numerical study of flow performance has been done. The main parameters to change in different cases are cone angle of the gas pipeline, α and depth of the pipeline in the gas cavity, L. By comparing the gas pressure drop, Δp and the gas outlets velocity nonuniformity, Su, we found that the case with α = 5° and L = 370mm is the best configuration. Comparing with the worst case, it may greatly decrease the velocity nonuniformity by 73.3% while the corresponding pressure drop increases only 8%.Copyright