Qiuyang Chen

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.

Review of Improvements on Shell-and-Tube Heat Exchangers With Helical Baffles

Helical baffles are employed increasingly in shell-and-tube heat exchangers (helixchangers) for their significant advantages in reducing pressure drop, vibration, and fouling while maintaining a higher heat transfer performance. In order to make good use of helical baffles, serial improvements have been made by many researchers. In this paper, a general review is provided of developments and improvements on helixchangers, which includes the discontinuous helical baffles, continuous or combined helical baffles, and the combined multiple shell-pass helixchangers. Extensive results from experiments and numerical simulations indicate that these helixchangers have better flow and heat transfer performance than the conventional segmental baffled heat exchangers. Based on these new improvements, the conventional heat exchangers with segmental baffles might be replaced by helixchangers in industrial applications to save energy, reduce cost, and prolong the service life and operation time.

A CFD-Taguchi Combined Method for Numerical Investigation of Natural Convection Cooling Performance of Air-Core Reactor with Noise Reducing Cover

High voltage air-core reactors are extensively used as components in electrical power distribution systems for many purposes. The noise of an air-core reactor can be reduced significantly using noise reducing cover. In the present article, the natural convection cooling performance of an air-core reactor with and without noise reducing cover is investigated to prevent the overheating problem. Six parameters, which all have five levels (values) of noise reducing cover, are considered. A Taguchi method is used as a systematic approach to plan the computational fluid dynamics (CFD) studies on the natural convection cooling performance. The optimum levels of six controllable parameters of noise reducing cover are determined. The relative contribution ratios of each parameter to the natural convection cooling performance are obtained. The contribution ratio of X 1, the radius of the bottom opening of noise reducing cover, is the most significant one. This allows design efforts to be concentrated on the most sensitive factors. The present CFD-Taguchi combined method can be applied and conducted quickly and inexpensively to plan a more reliable and robust optimization.

Numerical Studies of a Novel Combined Multiple Shell-Pass Shell-and-Tube Heat Exchanger With Helical Baffles

In order to simplify the manufacture and improve the heat transfer performance, we have invented a combined multiple shell-pass shell-and-tube heat exchanger. The novel combined multiple shell-pass shell-and-tube heat exchanger (CM-STHXs) with continuous helical baffles in the outer shell-pass and other different baffles in the inner shell-pass was compared with conventional STHX with segmental baffles by Computational Fluid Dynamics method. The numerical results show that, under the same mass flow rate M and the same overall heat transfer rate Qm in the shell side, the CM-STHX with discontinuous helical baffles in the inner shell-pass has the lowest overall pressure drop DP, which was 13% lower than that of the segmental baffled STHX; The heat transfer rate Qm of the CM-STHX with discontinuous helical baffles in the inner shell-pass is about 2% and 12% higher than those of the CM-STHX with segmental baffles and disk-and-doughnut baffles in the inner shell-pass. The CM-STHX with discontinuous helical baffles in the inner shell-pass has a much better heat transfer performance and can be used to replace the conventional STHX with segmental baffles in industrial applications to save energy, reduce cost and prolong the service life.

Thermal Design of Heat Exchanger With Fins Inside and Outside Tubes

Compact heat exchangers such as tube-fin types and plate-fin types are widely used for gas-liquid or gas-gas applications. Some examples are air-coolers, fan coils, regenerators and recuperators in micro-turbines. In this study, thermal design of fin-and-tube (tube-fin) heat exchanger performance with fins being employed outside and inside tubes was presented, with which designed plate-fin heat exchanger was compared. These designs were performed under identical mass flow rate, inlet temperature and operating pressure on each side for recuperator in 100kW microturbine as well as specified allowable fractions of total pressure drop by means of Log-Mean Temperature Difference (LMTD) method. Heat transfer areas, volumes and weights of designed heat exchangers were evaluated. It is shown that, under identical heat duty, fin-and-tube heat exchanger requires 1.8 times larger heat transfer area outside tubes and volume, 0.6 times smaller heat transfer area inside tubes than plate-fin heat exchanger. Under identical total pressure drop, fin-and-tube heat exchanger requires about 5 times larger volume and heat transfer area in gas-side, 1.6 times larger heat transfer area in air-side than plate-fin heat exchanger. Total weight of fin-and-tube heat exchanger is about 2.7 times higher than plate-fin heat exchanger, however, the heat transfer rate of fin-and-tube heat exchanger is about 1.4 times larger than that of plate-fin heat exchanger. It is indicated that, both-sides finned tube heat exchanger may be used in engineering application where the total pressure drop is severe to a small fraction and the operating pressure is high, and may be adopted for recuperator in microturbine.© 2006 ASME

Shell-and-Tube Heat Exchangers With Helical Baffles

In order to simplify the manufacture and improve the heat transfer performance, we have invented a combined multiple shell-pass shell-and-tube heat exchanger. The novel combined multiple shell-pass shell-and-tube heat exchanger (CM-STHXs) with continuous helical baffles in the outer shell-pass and other different baffles in the inner shell-pass was compared with conventional STHX with segmental baffles by Computational Fluid Dynamics method. The numerical results show that, under the same mass flow rate M and the same overall heat transfer rate Qm in the shell side, the CM-STHX with discontinuous helical baffles in the inner shell-pass has the lowest overall pressure drop DP, which was 13% lower than that of the segmental baffled STHX; The heat transfer rate Qm of the CM-STHX with discontinuous helical baffles in the inner shell-pass is about 2% and 12% higher than those of the CM-STHX with segmental baffles and disk-and-doughnut baffles in the inner shell-pass. The CM-STHX with discontinuous helical baffles in the inner shell-pass has a much better heat transfer performance and can be used to replace the conventional STHX with segmental baffles in industrial applications to save energy, reduce cost and prolong the service life.

The Comparison of Two Species Film Cooling Characteristics Between Trenched and Shaped Holes

The exit-shaped holes can result in lower coolant momentum injection with greater surface coverage. The exit-trenched holes can also lower the coolant momentum. Thus, the cooling and aerodynamic performance of laterally diffused shaped holes and laterally trenched holes were numerically compared with same depth and same hole length and the reasons for the difference were also analyzed from the viewpoint of flow mechanism. The both end-shaped holes and both end-trenched holes were also compared to the exit-shaped holes and exit-trenched holes respectively. Owing to the better heat transfer performance of steam than that of air, the cooling characteristics of super heated vapor film and pure air film were numerically investigated using the multi phase model of FLUENT to study the effect of different vapor volume fraction on film cooling characteristics. It appears that the shaped holes is superior to the trenched holes in cooling and aerodynamic performance for the cases in the present study; for shaped holes, the difference between the exit-shaped hole and both end-shaped hole is negligible; But for trenched holes, the cooling effectiveness of both end-trenched hole and the exit-trenched holes is heavily dependent on the hole length to diameter ratio; for shorter hole length to diameter ratio, the cooling effectiveness of both end-trenched hole is superior to that of exit-trenched hole. For all the cases studied, the mixture injectant is better than pure air coolant, and the mixture exhibits greater cooling advantage in the far downstream region of the holes than in the near hole region. The super heated vapor film can improve the film cooling effectiveness; the vapor volume fraction increased by 20%, and the area average cooling effectiveness can increase by 5%.Copyright

Film Cooling From a Row of Holes With Both Ends Embedded in Transverse Slots

Embedding a row of typical cylindrical holes in a transverse slot can improve the cooling performance. Rectangular slots can increase the cooling effectiveness but is at the cost of decreasing of discharge coefficients. An experiment is conducted to examine the effects of an overlying transverse inclined trench on the film cooling performance of axial holes. Four different trench configurations are tested including the baseline inclined cylindrical holes. The influence of the geometry of the upstream lip of the exit trench and the geometry of the inlet trench on cooling performance is examined. Detailed film cooling effectiveness and heat transfer coefficients are obtained separately using the steady state IR thermography technique. The discharge coefficients are also acquired to evaluate the aerodynamic performance of different hole configurations. The results show that the film cooling holes with both ends embedded in slots can provide higher film cooling effectiveness and lower heat transfer coefficients; it also can provide higher discharge coefficients whilst retaining the mechanical strength of a row of discrete holes. The cooling performance and the aerodynamic performance of the holes with both ends embedded in inclined slots are superior to the holes with only exit trenched. To a certain extent, the configuration of the upstream lip of the exit trench affects the cooling performance of the downstream of the trench. The filleting for the film hole inlet avail the improvement of the cooling effect, but not for the film hole outlet. Comparing film cooling with embedded holes to unembedded holes, the overall heat flux ratio shows that the film holes with both ends embedded in slots and filleting for the film hole inlet can produce the highest heat flux reduction.Copyright

Performance Optimization of Compact Ceramic High Temperature Heat Exchangers

In the present paper, the compact ceramic high temperature heat exchangers with parallel offset strip fins and inclined strip fins (inclined angle β = 0∼70°) are investigated with CFD method. The numerical simulations are carried out for high temperature (1500°C), without and with radiation heat transfer, and the periodic boundary is used in transverse direction. The fluid of high temperature side is the standard flue gas. The material of heat exchanger is SiC. NuS-G.R (with surface and gaseous radiation heat transfer) is averagely higher than NuNo.R (without radiation heat transfer) by 7% and fS-G.R is averagely higher than fNo.R by 5%. NuS-G.R (with surface and gaseous radiation heat transfer) is averagely higher than NuS.R (with only surface radiation heat transfer) by 0.8% and fS-G.R is averagely higher than fS.R by 3%. The thermal properties have significantly influence on the heat transfer and pressure drop characteristics, respectively. The heat transfer performance of the ceramic heat exchanger with inclined fins (β = 30°) is the best.© 2007 ASME