Numerical Simulation and Optimization on Air-Cooled Unit with Swirl Flow

CHENG You-liang School of Energy power and Mechanical Engineering,

North China Electric Power University, Baoding 071003, P. R. China

Email: ylcheng@live.cn

GUO Fei XUE Wei-peng School of Energy power and Mechanical Engineering,

North China Electric Power University, Baoding 071003, P. R. China

Email: yqgf@163.com

Abstract—The numerical simulation of conventional air-cooled

unit and a new air-cooled one in which the incoming air is

swirled by adding a swirler under the different angles of vane in

the swirler has been made by employing actual data of Longshan

power plant based on FLUENT software. Flow fields of the two

kinds of units are obtained and compared. The most appropriate

angle of vane in the swirler for heat transfer is found out.

Optimums of heat load and heat transfer area are obtained and

validated eventually by changing the head load and heat transfer

area in the numerical simulation.

Keywords- swirled air flow; air-cooled unit; numerical

simulation; optimization

I. INTRODUCTION

The problem of water shortage has been becoming more and more serious around the world. The technology of air-cooled condenser has widely used in thermal power plants due to its efficient water-saving performance. It has been nearly 70 years since the appearance of first air-cooled condenser in Germany. The types of air-cooled condenser include direct, indirect and dry-wet ones. The Northwest of China is rich in coal but lack of water, where the most of thermal power plants are more suitable for adopting the direct air-cooled condenser in order to mitigate the pressure of water shortage on it. [1-4].

The technology of air swirling has widely used in petroleum, chemical and agriculture [5-6] and has gotten many developments at home and abroad in recent years.

Experimental results of air swirling flow field have been obtained by using some measurement methods, like multi-tube pressure probe, PIV and so on [7-8]. In the condition of pipe flow, the heat transfer process with presence of weak swirling flow and turbulence statistics of isothermal air swirling flow have been obtained [9-10] and in the condition of unconfined swirling flow, flow field of reacting and non-reacting have been studied [11].

It were reported that enhancement of air-side disturbance can increase the heat exchange for direct air-cooled condenser in the literature [12-13]. In order to attain this purpose, enhancement of air velocity is always the first choice for general which means increase the power consumption of axial flow fans. Ventilation of air swirling flow can be applied to achieve the purpose of increasing the heat exchange and reduce the power consumption because its characteristics include high diffusion and disturbance.

In this paper, the flow fields of conventional air-cooled unit and the air-cooled one with vane swirler are simulated and compared using the FLUENT software. The heat transfer coefficient of air cooled units with different angle of vane swirler is obtained. Optimal result of heat load and heat transfer area is obtained and validated by changing the head load and heat transfer area in the numerical simulation.

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II. PHYSICAL MODEL AND BOUNDARY

CONDITIONS

A. Physical model

In order to ensure the accuracy of the simulation results, the geometry parameters of the air-cooled condenser unit used in this paper are based on actual size of Longshan power plant. There are two physical models employed in this study: the conventional air-cooled unit and the new air-cooled ones with a swirler under the different angles of the vane in it. The new air-cooled units with a swirler is that added by a air swirl device which have different angle of vane between axial flow fan and finned tubes, as seen in Figure 1.

Figure1. The new air-cooled condenser unit with an air swirler

Figure2. Air swirl device

Figure 2 gives the physical model of an air swirl device. For convenience, there is only one vane plotted in it.

B. Boundary conditions

Material is air and its density is 1.225kg/m3. In order to

avoid inconsistent with the actual operating conditions in the plant, the operating parameter in the spring which temperature of inlet boundary t2=288.16K, heat load Q=15229410W and heat transfer area A=26813m2 is employed in this paper. Except the inlet boundary is the fan’s inlet boundary and the outlet boundary is the outflow boundary, four faces around the control body are symmetry boundary. Finned tubes employ porous zone in which porosity is 0.567 and solid material is aluminum. Pressure-velocity term is iterated by SIMPLE algorithm, momentum equation is solved by first order implicit formulation, first order upwind format. Take 40° as an example of the angle of vane in the swirler, the mesh is shown in Figure 3.

Figure3. Sketch of mesh

III. NUMERICAL SIMULATION AND ANALYSIS

OF RESULTS

(a) The air-cooled condenser unit with a swirler

(b) Conventional air-cooled unit

Figure4. Sketch of velocity vector (face y=0)

Velocity vector (face y=0) within the new air-cooled condenser unit with a swirler and the conventional air-cooled one are simulated and their sketches are shown in Figure 4, take 40°as an example in Figure 4 (a).

As shown in the Figure 4, it should be that there are important differences between the new unit and the conventional one in the air flow field. The velocity vector of the new unit shown in the Figure 4 (a) exhibits symmetric distribution and typical characteristics of swirl flow, however, the velocity vector of conventional one given in Figure 4 (b) exhibits uneven state due to influence of tangential velocity of fan blade. Moreover, the Figure 4 (a) shows that air flow in the new unit towards the finned tubes lead to the velocity larger than that of the conventional one near the finned tubes due to strong diffusion of swirling air flow. It can be observed that the air flow out from the finned tubes in the Figure 4 (a) is more affluent than that in the Figure 4 (b). It can also be seen from the Figure 4 (a) that the center-back flow which is the most typical characteristic of swirling air flow.

Figure5. Tendency of average temperature in the outlet on the airside of

the finned tubes under the different angles of vane in the swirler

The tendency of average temperature (t2) in the outlet on the airside of the finned tubes in the air-cooled condenser unit under the different angles of vane in the swirler with a 5° interval is given in Figure 5. The temperature t2 of the conventional air-cooled unit is 310.8673K from numerical simulation. From Figure 5 it can be seen the tendency of decreasing before the 40°is slower than the tendency of increasing after the 40°, especially even slower than the tendency of increasing from 65° to 75°; The minimum temperature t2 is attained when the angle of the vane is 40°and is 309.9062K. The main reason is that relatively weaker swirling flow can increase heat transfer, the swirl intensity enhances with the increase of the vane angle and hence optimum vane angle can be achieved; Deterioration of heat transfer can be generated after the 40°due to stronger diffusion and shorter jet distance length with enhancement of the swirl intensity as shown in Figure 5.

IV. OPTIMIZATION AND VALIDATION

Figure6. Tendency of heat transfer coefficient of the new air-cooled

condenser unit under the different angles of vane in the swirler

From Figure 6 it can be seen that the tendency of heat transfer coefficient (k) is same as the tendency of temperature t2 given in Figure 5. The heat transfer coefficient k of conventional air-cooled unit is 20.2883W/m2·K which is calculated using the result of simulation. In order to make the performance of heat transfer full play, optimums of heat load and heat transfer area are calculated employing the heat transfer coefficient k of conventional air-cooled unit and the result is obtained and given in Figure 7 and Figure 8.

Figure7. Optimum of heat load of the new air-cooled condenser unit

under the different angles of vane in the swirler

Figure8. Optimum of heat transfer area of the new air-cooled condenser

unit under the different angles of vane in the swirler

In order to ensure the accuracy of optimum of heat load and heat transfer area, validation is carried out by changing the head load and heat transfer area in numerical simulation. The comparison is implemented between k which is calculated by the modified heat load and heat transfer area and k=20.2883W/m2·K which is calculated by the non-modified heat load and heat transfer area, and the max deviation value is 0.3199% for optimum of heat load and 0.8872% for optimum of heat transfer area. The deviation value would not large enough and could be ignored and hence the accuracy of optimum should be certificated.

V. CONCLUSIONS

Based on FLUENT software, the numerical simulation has been carried out for the conventional air-cooled unit and the new air-cooled one added by a swirler. The following conclusions have been obtained:

The flow field of the air-cooled unit with swirler exhibits typical characteristics of swirling flow and is more affluent than conventional one when air flow is through the finned tubes;

The minimum of the temperature t2 in the outlet on the airside of the finned tubes can be achieved when the angle of vane in the swirler is 40°due to swirl intensity of 40° is most

appropriate for heat transfer of unit according to the numerical simulation;

Optimums of heat load and heat transfer area are obtained by calculating. Accuracy of optimum is validated by changing the head load and heat transfer area in numerical simulation and hence the accuracy is certificated.

ACKNOWLEDGMENTS

The authors are grateful to the support from the National Natural Science Foundation of China (Grant NO.10672056)

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