Cao Zhenheng 1,2, Yan Hongwen 3, Liu Shufan 3, Gao Xiaoming 1,2, Fu Feng 1,2 References: slightly
(1 Key Laboratory of Chemical Reaction Engineering of Shaanxi Province, Yan'an 716000, China; 2 College of Chemistry and Chemical Engineering, Yan'an University, Yan'an 716000, China; 3 Shaanxi Yanchang Petroleum (Group) Co., Ltd., Xi'an 710075, China)
Abstract: A new type of heat transfer strengthening component, multi-face baffle, was designed and fabricated for heat transfer enhancement in tubular heat exchanger tubes. The relationship between the convective heat transfer Nusselt number, the resistance loss, the comprehensive performance evaluation factor and the Reynolds number in the heat exchanger tube after the baffle reinforcement of two sizes is preliminarily determined, and compared with the blank experiment, the experiment shows that: multi-faceted The baffle can significantly improve the heat transfer performance of the tubular heat exchanger, and can improve the convective heat transfer coefficient in the pipe by 55% to 100%. The strengthening effect is related to the structural parameters of the multi-face baffle. The comprehensive performance evaluation factor of the baffle 145 is greater than 1 in the experimental range. When Re=20,000 to 21 000, ηmax=1.156, the comprehensive performance of the heat exchanger is significantly improved compared with the non-reinforced.
Key words: multi-face baffles; field synergy; comprehensive performance evaluation factor; tubular heat exchanger; internal reinforcement
Heat exchangers are indispensable equipment in many industrial productions and are widely used in petrochemical, coal, light industry, aerospace and other engineering fields. In heat exchangers of various forms, tubular heat exchangers still dominate. Heat transfer enhancement of tubular heat exchangers has long been one of the research hotspots in the engineering field.
A simple way to reduce the heat transfer resistance of the convection heat transfer of the heat exchanger is to increase the turbulence by using a reinforcing element, which mainly includes a threaded tube, a horizontal tube, a scale tube, a spiral fin tube, and a heat exchange tube used in the heat exchanger. The middle spoiler is used to strengthen the heat transfer inside the pipe. The addition of spoiler in the heat exchange tube for enhanced heat transfer has been used in the industry for many years. It can significantly improve the total heat transfer coefficient, greatly reduce the heat transfer area of ​​the heat exchanger, reduce the weight of the equipment, and save a lot of metal materials. . Many scholars at home and abroad are committed to the study of heat transfer enhancement in tubular heat exchangers [1-8]. In this paper, a new heat transfer enhancement component, multi-face baffle, is used as an internal plug-in. The strengthening effect and influence of the heat transfer performance of the heat exchanger.
1. Experiments and methods
1.1 Structure of multi-face baffles
Our research group designed and produced a new type of heat transfer enhancement component, multi-face baffle, which can be inserted into the tube heat exchanger tube as a heat transfer enhancement element. The experimental study on the heat transfer enhancement element to tube heat transfer The heat transfer performance, resistance performance and overall performance of the device.
The multi-faceted baffle is made of a rectangular metal piece of a certain size before the production of the multi-face baffle, and the multi-face baffle used for the cost experiment is folded in the same manner as shown in FIG.
In Fig. 1, ΔABC ≌ ΔCAD ≌ ΔDCE ≌ ... and are all isosceles triangles, wherein points A, C, D, etc. are the vertices of the above-mentioned isosceles triangle.
After the inner reinforcing element is inserted into the tube, as shown in FIG. 2, the apexes of the baffle unit triangles are respectively inscribed on the circular surface of the inner wall of the tube of the heat exchanger. The projections of the vertices of the triangular elements of the multi-face baffles in the direction of the tube axis respectively overlap, that is, the ΔA'B'C' obtained by the axial projection is an equilateral triangle, and the arcuate gap between the triangle and the inner wall allows the fluid to pass freely.
The baffle is placed in a circular straight tube of a tubular heat exchanger, and after the fluid enters the tube in the axial direction, the flow field of the fluid in the tube will change significantly under the forcing action of the baffle of the multi-face baffle, part of the fluid Flow around the fold surface in a certain way (see Figure 3), thereby pushing and strengthening the turbulence of the fluid at the tube wall, reducing the thermal resistance of the convection heat, and achieving the purpose of enhancing heat transfer.
By changing the angle of the apex angle of the polygon of the multi-face baffle unit, that is, changing the angle of the apex angle ∠BAC of ΔABC in FIG. 1, the purpose of changing the dimensional parameters such as the degree of distortion of the multi-face baffle is obtained, thereby comparing the folds of different size structures. The heat transfer enhancement effect of the flow sheet on the tube heat exchanger.
In this experiment, the internal heat transfer performance of two different-shaped multi-faceted baffles with ∠BAC equal to 60o and 145o respectively was determined. For convenience, the following two kinds of multi-face baffles are: baffle 60, The baffles 145 were compared to the blank experiment, i.e., without baffles.
1.2 Experimental methods
The heat exchanger used in the experiment was a tube-type heat exchanger, the inner tube was a copper tube, and the outer tube was a stainless steel tube. The cold fluid air travels through the tube, the hot fluid water moves away from the shell, and the countercurrent heat exchange. The structural parameters of the casing heat exchanger are shown in Table 1. In the experiment, the parameters such as controlling the flow rate of air and water, temperature and other parameters were kept stable, relevant data were measured, and the experimental data were processed, analyzed and compared.
2. Results and discussion
According to the method described in Section 1.2, the experiment is carried out, and the original experimental data is processed to obtain the Reynolds number Re, the convective heat transfer Nusselt number Nu, the flow resistance loss hf and the integrated in the tube heat exchanger. Data such as the performance evaluation factor η are summarized in Table 2.
2.1 Relationship between convective heat transfer Nusselt number and Reynolds number According to the data in Table 2, the Nusselt number Nu of the convective heat transfer of the inner tube of the casing heat exchanger is plotted with its flow Reynolds number Re, as shown in Fig. 4.
As can be seen from Fig. 4, both sizes of baffles can significantly increase the Nusselt number Nu of the convective heat transfer in the casing heat exchanger. As can be seen from the data in Fig. 4 and Table 1, the baffles 60, The baffles 145 can increase the convective heat transfer coefficient in the tube by 98% to 100% and 55% to 95%, respectively. This is because when the fluid flows through the multi-face baffle, it promotes the fluid to be similar to the spiral flow, effectively controls the development of the boundary layer, reduces the thickness of the laminar inner layer, reduces the heat transfer resistance inside the tube, and enhances the heat transfer. . Moreover, the smaller the apex angle of the baffle unit triangle, the greater the distortion of the baffle, the more severe the disturbance of the fluid in the tube, and the radial flow is intensified, so that the thickness of the laminar inner layer at the tube wall is further reduced. Therefore, there are Nu baffles 60>Nu baffles 145>Nu no baffles, and the larger the Reynolds number Re of the fluid flow in the tube (the larger the air flow rate), the more remarkable the multi-face baffle heat transfer enhancement effect.
2.2 Relationship between fluid resistance loss and Reynolds number
According to the data in Table 2, the flow resistance loss hf of the heat exchanger inner tube is plotted against the flow Reynolds number Re, as shown in Fig. 5.
It can be seen from Fig. 5 that the addition of the baffles causes the resistance loss of the inner tubes of the heat exchanger to increase to different extents, and the hf baffles 60>hf baffles 145>hf have no baffles. The larger the Reynolds number Re, the larger the resistance loss hf, but the resistance loss of the heat exchanger after the baffle 60 is added increases with the Reynolds number much faster than the blank of the baffle 145 and the baffleless sheet. In the experiment, the resistance of the fluid when it is similar to the spiral flow can be divided into the frictional resistance caused by the viscous action and the physical resistance caused by the change of the flow direction. The smaller the triangle apex angle of the multi-face baffle unit, the greater the distortion of the baffle, and the greater the angle between the baffle surface and the flow direction of the fluid body, so that the body resistance also increases rapidly.
2.3 The relationship between comprehensive performance evaluation factors and Reynolds number
As can be seen from Fig. 4 and Fig. 5, the smaller the triangular apex angle of the multi-face baffle unit, the greater the distortion of the baffle constructed, the more the heat transfer coefficient increases, and the more the resistance loss of fluid flow is. Large, so it is impossible to judge the overall performance of that type of baffle. In this paper, the comprehensive performance evaluation factor η is used to describe the comprehensive performance of multi-face baffle heat transfer enhancement [9], which is defined as equation (1).
In the formula, Nu0 and f0 are the heat transfer Nusselt number and fluid flow resistance coefficient of the inner tube of the heat exchanger without strengthening; Nu and f are the heat transfer Nusselt number and fluid flow resistance of the inner tube of the heat exchanger after strengthening. coefficient. The comprehensive performance evaluation factor is based on the non-reinforced tube. η>1 indicates that the comprehensive performance of the strengthening element is strengthened compared with no strengthening. Generally speaking, the larger the η, the better the comprehensive performance. According to the data in Table 2, the comprehensive performance evaluation factor η for the heat transfer enhancement of the two kinds of multi-face baffles used in this paper is plotted with the flow Reynolds number Re in the tube, as shown in Fig. 6.
As can be seen in Fig. 6, the overall performance evaluation factor curve of the baffle 145 is above η = 1, indicating that the multi-faceted baffle improves the overall heat transfer performance of the tube. The comprehensive performance evaluation factor η of the baffle 60 is less than 1, indicating that although the multi-face baffle can enhance heat transfer, the resistance increases too much, resulting in poor overall performance.
The comprehensive performance evaluation factors of the baffle 60 and the baffle 145 reach a maximum at a Reynolds number Re of about 20 000, respectively, indicating that there is a suitable flow Reynolds value for the heat transfer enhancement elements. The special multi-planar structure of the multi-face baffle basically conforms to the field synergy principle [10-11] to optimize the heat transfer performance of the heat exchanger.
3 Conclusion
The two multi-face baffles (baffles 60 and baffles 145) used herein can increase the convective heat transfer coefficient in the casing heat exchanger tubes by 55% to 95% and 98% to 100%, respectively. In the experimental range, the comprehensive performance evaluation factors of the baffles 145 are all greater than 1. When Re=20 000-211,000, the comprehensive performance evaluation factor η reaches a maximum value of about 1.156, and the comprehensive performance of the heat exchanger is significantly improved.
The simple and special structure of the multi-face baffle can just exert its better field synergy effect, which makes the comprehensive performance of the heat exchanger significantly improved.
The structural parameters of the multi-face baffle play a decisive role in the overall performance of the tubular heat exchanger. The baffle sheet strengthens the heat transfer mainly by using the fluid to flow around the baffle, forming a local low-resistance high-efficiency area downstream, which can effectively wash the wall surface and enhance the heat exchange. At the same time, this area is between the baffle and the tube wall. At the gap, there is no frictional resistance and shape resistance loss of the baffle, so the overall heat transfer performance of the baffle is effectively improved.
The multi-faceted baffle is a multi-planar structure formed by stacking and twisting triangular elements. It is simple in design and manufacture, and low in cost. Therefore, it has certain engineering practical value and is expected to be applied in related fields.