Understanding the Finite Element Analysis of Flat Plate Heat Exchanger

Finite element analysis of a heat exchanger is done in order to avoid the process of experimentation and improvement on physical products. The finite element analysis on the flat plate heat exchanger for waste heat recovery system will help analyse the performance of flat heat exchanger and make improvement in design or operating parameters to get best result possible. 

The finite element analysis done by T. Kho (1999) on a flat plate heat exchanger to understand the flow of fluid on the plate during the operation involves four different shapes of plates. The main focus of T. Kho (1999) was to analyse the effect of parameters like velocity and shape of the flat plate used, on the flow of liquid in flat plate heat exchanger. His work involves plate of different geometry which includes four different profiles as shown below. T. Kho (1999) concluded that during working of flat plate heat exchanger velocity of fluid with which it flow in channels of flat plate heat exchange is the most important factor in deciding the occurring of fouling. The work concluded that low velocity with high temperature cause fouling in flat plate heat exchange and to avoid this redesigning of flat plate heat exchange plate is needed. T. Kho (1999) improves geometry of plates of flat plate heat exchange by placing the distributers at different places of plate. This diverge the flow of fluid which help to avoid fouling in plate heat exchange.

Figure 10 plate with diverters T. Kho (1999)

This experimental work predict that to make fluid flow more smooth in a flat plate heat exchange, the adjacent corners of the flat plates should be made circular rather than with sharp edged. This smooth curve corner will allow more evenly distribution of fluid flow over the plates. The effect of distributors and the effect of smooth corner on the flow of the fluid and temperature distribution is shown below. The figure show that the smoother the corner are smoother will be the distribution of flow on the plates of flat plate heat exchange and greater will be the distribution of temperature all over the plate of flat plate heat exchange.

Figure 11 flow and temperature distribution with diverters T. Kho (1999)

Flavio (2006) work to perform experimental and numerical investigation of flat plate heat exchanger in order to understand the role of CFD in predicting the heat transfer in flat plate heat exchanger using a 3 D model. This work makes use of small flat plate heat exchanger where plate of 90 mm by 60 mm in length and width were used during the experiment and simulation. Thickness of plate of flat plate heat exchanger was 1 mm and the heat transfer area available was 0.005 meter square per plate with total of 3 plates used for this process. Flavio (2006) make use of counter concurrent Z type flow in flat plate heat exchanger experiment where experiment was conducted in 2D and 3D model. Results from the analysis shows transfer of temperature from hot fluid to cold fluid when hot fluid moves between hot fluid channels. The temperature profile shows decrease in temperature of hot fluid as fluid moves from A and C plate. It also show how the diagonal corner of the plate of flat plate heat exchanger has lowest temperature recorded. Similarly the plate B and C of flat plate heat exchanger shows the increase in temperature of cold fluid this also show the same phenomena where diagonal corner of the plate of flat plate heat exchanger has lowest temperature recorded.

Figure 12 Temperature pattern in plate heat exchanger Flavio (2006)

 

This difference is temperature value in an individual plate of flat plate heat exchanger is explained by the fluid flow pattern of flat plate heat exchanger. As shown in the figure below the flow of hot and cold fluid in the opposite corners of the inlet is almost zero. If hot and cold fluid does not reach that point then the heat transfer process in those corners will not occur. This is the reason that temperature in those regions is very low.

Figure 13 velocity pattern in plate heat exchanger Flavio (2006)

Pressure drop across the flat plate heat exchanger is mainly due to the resistance face by the fluid while flowing in the channels of the flat plate heat exchanger. The resistance faced by the fluid usually dependent on the type of flow, the flow rate and the geometry of the plate used in the flat plate heat exchanger. According to the study of the Funke (2019) the literature available on the pressure drop of the flat plate heat exchanger has very large difference for different authors and some inaccuracies as well. Funke (2019) work briefly discusses the pressure drop across the flat plate heat exchanger. It was concluded in that work that the turbulent flow increases the fluid ability to transfer heat. This increase in fluid ability to transfer more heat in its turbulent state is due to the fact that turbulent flow enables fluid mixing between different layers of the fluid which give better temperature average in fluid as well as greater fluid areas for heat transfer. This increase heat transfer due to turbulent flow comes at the cost of higher pressure drop across the heat exchanger. Higher pressure loses across the heat exchanger means more work input at the pump is required. The pressure drop across the flat plate heat exchanger can be calculated as follow (Funke, 2019).

In above equation delta p represent the pressure drop across the flat plate heat exchanger, f represent the fanning friction factor or also called the fanning factor of flat plate heat exchanger channel, omega represent the average velocity of the fluid inside the channels of flat plate heat exchanger, L represent the effective length of the flat plate heat exchanger, row represent the density of the fluid of flat plate heat exchanger and D represent the equivalent diameter of the flat plate heat exchanger. In above equation the fanning factor of the flat plate heat exchanger is the main factor which decide the pressure drop the across the flat plate heat exchanger. If the fanning factor is measured correctly the pressure drop across the flat plate heat exchanger can be calculated with the precision of 50 % up to 100 %. The fanning factor of the flat plate heat exchanger totally depends on the geometry of the flat plate heat exchanger so the fanning factor calculated for a typical heat exchanger is only applicable for that flat plate heat exchanger.

As explained earlier that the mass flow rate and the plate geometry of flat plate heat exchanger are the two main factor effecting the pressure drop of heat exchanger and the work of Aydin  (2009) show the effect of plate geometry and mass flow rate of on the pressure drop of the flat plate heat exchanger.  Work involves the comparison of three different plates of flat plate heat exchanger each have different geometry in terms of the plate face where fluid will flow. First type of the plate of flat plate heat exchanger has a flat face for both hot and cold fluid channel, the second type of plate of flat plate heat exchanger has a corrugated face and third type of plate of flat plate heat exchanger has an asterisk face. The face of the corrugated plate of flat plate heat exchanger shown below is made with the help of die which extrude the material from one side of the plate face to get corrugated feature and impression on the other opposite face of the plate. Asterisk face of the third plate of flat plate heat exchanger is also manufactured in very similar manner where start like shapes where manufactured using a die to press manufactured the star shape on plate. Plates used in flat plate heat exchanger by the Aydin (2009) have rectangular cross section with holes present at each corner of the plate to enable hot fluid and cold fluid to enter and exit the flat plate heat exchanger.

Figure 14 asterisk and corrugated plate for Heat exhanger Aydin (2009)

The comparison made by Aydin (2009) show that the energy loss in the corrugated plate flat plate heat exchanger is much more as compared to that of the asterisk plate flat plate heat exchanger and the energy loss at asterisk plate flat plate heat exchanger is much more as compared to that of the flat plate of flat plate heat exchange. This greater loss of energy at the corrugated plate flat plate heat exchanger is due to the greater resistance faced by the fluid during its flow inside channels of corrugated plate flat plate heat exchanger. Greater the resistance made by the plate of flat plate heat exchanger more turbulent will be the flow and turbulent flow of the fluid requires greater force or power to flow. Due to this reason the turbulent flow of the fluid have greater energy loss as compared to the laminar flow at flat plate of flat plate heat exchanger. In Aydin (2009) work finds that greater the turbulent flow is greater is the heat transfer happening inside the flat plate heat exchanger. So the heat transfer in corrugated plate flat plate heat exchanger is much more as compared to that of the asterisk plate flat plate heat exchanger and the heat transfer at asterisk plate flat plate heat exchanger is much more as compared to that of the flat plate of flat plate heat exchange. Aydin (2009) also worked to find the effect of mass flow rate on the energy losses, pressure losses and the heat transfer in flat plate heat exchanger. According to work the increase in mass flow rate of the hot fluid enables the flat plate heat exchanger to have better heat transfer from hot fluid to cold fluid whereas the same increase in mass flow rate also increases the energy loss in flat plate heat exchanger. This explained by the fact that increases in mass flow rate of the hot fluid in flat plate heat exchanger increases the turbulent nature of the flow which enable better heat transfer and greater pressure loss in flat plate heat exchanger.

Galezzo (2006) used computational fluid mechanics CFD to solve the flat plate heat exchanger with the help of ANSYS fluent software. Work starts with the development of the cad model inside the ANSYS software which was then meshed and solve for heat and momentum transfer.  T.Kho (1992) makes use of computational fluid mechanics in ANSYS CFX to solve the same heat and momentum transfer equation for the flat plate heat exchanger. Work pattern of T.Kho (1992) was very similar to that of the Galezzo (2006) where work started with the computer added model, meshing and then finally the solution of the flat plate heat exchanger.