Design and performance analysis of a discontinuous spiral baffle heat exchanger

In this paper, a new type of heat exchanger with discontinuous spiral baffles is designed, and its performance on heat transfer is analyzed by numerical simulation. The research results show that the airflow in shell-side flows by a similar helical way, “Flow dead zone” is not found. The high enthalpy airflow’s temperature decreases about 38% of the inlet air temperature by cooling down with a convective heat transfer coefficient of 344.4 W/(m2·K), while the decrease of the airflow pressure is 30%.

heat transfer Coefficient and pressure drop of head-tail joint with small angle of 30 ° are higher than the degree of overlap is 40% and 50% respectively with large angle of 40 °.Chen Yaping [8] found that the outer notch opens a short circuit directly from the upstream channel to the downstream channel, which is not conducive to the flow heat transfer of the spiral main stream.
At present, the research on spiral baffle heat exchangers mainly focuses on how to further improve the heat exchange performance of spiral baffle heat exchangers from the perspective of structural design, and rarely considers whether the designed spiral baffle is easy to process.In this paper, a heat exchanger whose baffle is easy to process is designed and its performance is investigated.Firstly, the heat transfer coefficient, heat exchange area and other parameters of the heat exchanger are calculated according to the engineering design, and then the structure of the heat exchanger and the number of heat exchange tubes are determined.Secondly, the performance of heat exchanger is simulated numerically by fluent software.

The design of discontinuous spiral baffle heat exchanger
At present, the traditional shell-and-tube heat exchanger design has a complete set of engineering design methods which can be adopted, but for the compact heat exchangers, up to now, there is not a complete and systematic engineering design method to be used for reference.
In view of the fact that there is no ready-made engineering design method available, this paper intends to use a method combining engineering calculation and numerical simulation to design and analyze the performance of the discontinuous spiral baffle heat exchanger studied.
Firstly, the heat transfer coefficient, heat transfer area and other parameters of the heat exchanger are calculated by engineering design method according to the index requirements.
Secondly, the results of engineering calculation are used to guide the structural design of heat exchanger.
Finally, commercial software is used to model the designed heat exchanger, divide the grid, and perform numerical simulation.If the design requirements cannot be met, it is modified, and then renumerical simulation until the optimal solution is obtained.

Numerical simulation of discontinuous spiral baffle heat exchanger
The internal structure of the discontinuous spiral baffle heat exchanger is shown in Figure 1.The shape of the helical baffle heat exchanger designed in this paper is a rectangular cuboid with a rectangular cross section.The heat exchanger is composed of a shell, a tube bundle, a number of spiral baffles, a fluid inlet and outlet nozzle on the shell side and a fluid inlet and outlet nozzle on the tube side.The parameters of the heat exchanger designed according to the previous calculation process are as follows: the number of heat exchange tubes is 93, the diameter of the inlet and outlet tubes on the shell side is 40mm, the inclination Angle of the baffle is 15°, the pitch of the baffle is 134mm, and the baffle is righthanded.The baffle is rectangular in shape, simple in structure and easy to process.The four corners of the rectangular baffle are respectively bonded to form a discontinuous spiral baffle, as shown in Figure 1 (a).The fluid in the shell of the heat exchanger is hot air, and the fluid in the tube is kerosene.After calculation, the shell process Re 1 =36913 and the tube process Re 2 =13586 are both in a turbulent state, and the flow heat transfer is steady.The physical property parameters of kerosene and air are set as constant during calculation.The effect of gravity is ignored in the calculation.The inlet of air and kerosene is defined as a mass flow inlet and the air is set as an incompressible fluid.The exits are all pressure exits, and the outer wall of the heat exchanger is an adiabatic wall.show the total pressure distribution along X, Y and Z directions of the fluid air in the shell side of the heat exchanger.As can be seen in Figure 3 and Figure 5, the pressure of the shell-side fluid in the X and Z directions tends to decrease in the direction of the flow, and the pressure of the shell-side fluid remains basically constant in the area in front of each baffle, while after flowing through the baffle, there has been a marked drop in pressure.This shows that the helical baffle can effectively reduce the dead zone of the flow, but it still has a relatively strong disturbance to the flow of the fluid, therefore, the pressure loss caused by the existence of baffles in heat exchangers is inevitable.
As can be seen from the Y-plane total pressure distribution diagram in Figure 4.The pressure distribution in the radial section of the heat exchanger presents a spiral decreasing distribution, which indicates that the fluid also presents a spiral flow in the circumference.It can also be noted from the figure that the pressure on both sides of the baffle intercepted in the observation plane has an obvious step, while in the direction of the clockwise flow of the fluid, the pressure is gradually reduced, indicating that the existence of the baffle is also the main cause of pressure loss in the process of circumferential rotating flow.Figure 6.Velocity distribution along Y direction.Through analysis and calculation, the total pressure loss of the fluid in the shell is 32%, and the total pressure loss of the fluid in the tube is 2.9%.The pressure loss of shell flow is large, mainly because the shell of the heat exchanger is cuboid, the fluid flows helically in the rectangular flow section, and the corner angle of 90° will be generated at the boundary, and there will be a great momentum loss.

Fluid velocity distribution in shell side of heat exchanger.
Figure 6 shows the velocity distribution at the shell side of the heat exchanger along y-direction, v z is the velocity along the z-direction, v x is the velocity along the x-direction, and the velocity distribution in the shell side of the heat exchanger along the y-direction is xy v  .It can be seen that the circumferential velocity component v x and v y and the axial velocity component v z exist at the same time, which indicates that the fluid flow in the heat exchanger has both rotational flow and longitudinal flow.
The distribution of the axial velocity component v z can be seen from the velocity cloud diagram: (1) the velocity of the central part is higher than that of the surrounding part, which shows that the longitudinal flow of the heat exchanger is mainly completed in the middle part of the heat exchanger, the longitudinal velocity of the fluid near the wall of the heat exchanger is very small.(2) when the shell-side fluid flows to the baffle, the shell-side fluid is subjected to a large flow resistance, where the longitudinal flow velocity is very small, most of the fluid flows away from the gap at the side of the baffle, and a high-speed flow zone appears near the center.
The distribution of the circumferential velocity xy v  can be seen in the velocity vector diagram: (1) because of the guide effect of Baffle, there is obvious secondary flow in the heat exchanger, which makes the air scour the heat exchanger tube bundles vertically, and forms the staggered flow pattern, which is more advantageous to the full heat transfer of the cold and hot medium.
(2) at the center, the gap formed by the installation of the baffle causes the high-speed flow, the distribution of xy v  is consistent with that of the v z , while in the wall region of the heat exchanger, the v z is small but the value is still high, which indicates that the Longitudinal flow is hindered in this region, the spiral flow with higher velocity is formed, and there is no flow stagnation in the wall and corner of the heat exchanger, so the flow dead zone is effectively reduced.

Heat transfer performance analysis of discontinuous spiral baffle heat exchanger
The heat transfer coefficient of the whole heat exchanger is k = 344.4W/(M 2 • K) .The heat transfer performance meets the design requirements.
The distribution of temperature field in x and y directions of the heat exchanger is shown in Figure 7 and Figure 8 respectively.Here, the absolute temperature is dimensionless treated by the following formula, that is, the dimensionless temperature T is: It can be seen that: (1) the temperature of the air flow through the baffle decreases obviously, because the baffle leads the air to form a spiral secondary flow, which scours the heat exchange tube vertically and improves the heat exchange effect.(2) the fluid temperature near the central shaft of the Heat Exchanger is higher than that around it, because the area of the combined gap of the baffle plate is the largest near the central shaft, the Air Flow Velocity is the fastest, and the direction of the flow is basically parallel to the tube bundle, these reasons lead to the poor heat transfer effect and high temperature in such areas.

Conclusion
In this paper, a discontinuous helical baffle heat exchanger with wall cooling is studied by combining numerical simulation with engineering calculation.The conclusion is as follows: (1) The shell side fluid of the discontinuous spiral baffle heat exchanger flows in a spiral shape, which can effectively eliminate the "Flow dead zone" caused by the traditional baffle and enhance the heat transfer effect of the heat exchanger, the temperature drop of the air cooled by the heat exchanger is about 38% of the original air flow temperature, and the overall heat transfer coefficient of the heat exchanger is k = 344.4W/ (M 2 • k), to meet the design requirements; (2) The total pressure loss of the fluid air in the shell side is larger than that in the other side, up to 30%.Part of the pressure loss is caused by the shape of the shell of the heat exchanger cuboid, and the flow resistance is increased by the rectangular flow section, the other part of the loss is caused by the presence of spiral baffles.

Figure 1 .
Figure 1.Internal structure Diagram of discontinuous spiral baffle heat exchanger.The fluid in the shell of the heat exchanger is hot air, and the fluid in the tube is kerosene.After calculation, the shell process Re 1 =36913 and the tube process Re 2 =13586 are both in a turbulent state, and the flow heat transfer is steady.The physical property parameters of kerosene and air are

3. 1 .
Analysis of the flow in the discontinuous spiral baffle heat exchanger 3.1.1.Fluid flow analysis in Shell side of heat exchanger.Figure2is the trace diagram of the fluid-air flow in the shell side of the heat exchanger.It can be clearly seen from the figure that the shell fluid flows in a spiral shape under the action of the spiral baffle, and forms a vertical scouring flow mode against the tube bundle in most areas, which effectively enhances the effect of convective heat transfer between the air and the heat exchange tube.It can also be seen from the figure that the air flow under the action of the spiral baffle shape does not appear a relatively stagnant flow "flow dead zone".From this point of view, it also effectively increases the effect of the heat exchanger, thereby improving the convective heat transfer coefficient of the heat exchanger.

Figure 2 .
Figure 2. Air trace map.3.1.2.Flow pressure distribution in shell side of heat exchanger.Figures 3, Figures4 and Figures5show the total pressure distribution along X, Y and Z directions of the fluid air in the shell side of the heat exchanger.As can be seen in Figure3and Figure5, the pressure of the shell-side fluid in the X and Z directions tends to decrease in the direction of the flow, and the pressure of the shell-side fluid remains basically constant in the area in front of each baffle, while after flowing through the baffle, there has been a marked drop in pressure.This shows that the helical baffle can effectively reduce the dead zone of the flow, but it still has a relatively strong disturbance to the flow of the fluid, therefore, the pressure loss caused by the existence of baffles in heat exchangers is inevitable.As can be seen from the Y-plane total pressure distribution diagram in Figure4.The pressure distribution in the radial section of the heat exchanger presents a spiral decreasing distribution, which indicates that the fluid also presents a spiral flow in the circumference.It can also be noted from the figure that the pressure on both sides of the baffle intercepted in the observation plane has an obvious step, while in the direction of the clockwise flow of the fluid, the pressure is gradually reduced, indicating that the existence of the baffle is also the main cause of pressure loss in the process of circumferential rotating flow.
4 and Figures

Figure 3 .
Figure 3.Total pressure distribution along X direction.

Figure 4 .
Figure 4. Distribution of total pressure along Y direction.

Figure 5 .
Figure 5.Total pressure distribution along Z direction.

Figure 7 .
Figure 7. Dimensionless temperature field distribution diagram along X direction.

Figure 8 .
Figure 8. Dimensionless temperature field distribution diagram along Z direction.