Understanding the Spiral Heat Exchanger and its Finite Element Analysis

Spiral heat exchanger is a unique type of heat exchanger where two or more metal plates are rolled together into a spiral with some gap between plates. The spiral of two or more plates with gap between them creates two or more channel which enables the flow of two different fluids in heat exchanger (Vendat 2000). The spiral gives this type of heat exchanger a greater surface area to exchange heat between fluids at very compact and small space. This feature of spiral heat exchanger increases the overall thermal conductivity of the heat exchanger (Metta, Konijeti et al. 2018).  
Figure 4 spiral heat exchanger (Metta, Konijeti, 2018)

Types of Spiral heat Exchanger
There two main types of spiral heat exchanger where one is based on the number of channels a spiral heat exchanger has and other is based on the direction of flow of fluid in the channels (Vendat 2000). In the first type there can single channel spiral heat exchanger where there is one channel for each fluid to flow and then there can be a multi channels spiral heat exchanger where there can be more than one channel for a single type of fluid to flow. In the second type of spiral heat exchanger the two different types of fluid can flow in same direction to call the heat exchanger a parallel flow spiral heat exchanger or both fluids can flow in opposite direction to called the heat exchanger a counter flow spiral heat exchanger (Jamshid Khorshidi 2016).
Figure 5 types of flow of Spiral Heat Exchanger

Operating Conditions of Spiral Heat Exchanger
The spiral heat exchanger are designed for a verity of operations and based on the requirements they can be designed with custom specifications. The current status of spiral heat exchanger show that they are used for operation which can involve full vacuum pressure to pressure of about 80 bars and can work for the temperature range of -200 degree C to about 450 degree C. Due to their simple design they can be used for any type of fluid which can involve, water, steam, gasses and even coolants or refrigerants. 

Overall Heat Transfer coefficient of Spiral Heat Exchanger
According to the working of spiral heat exchanger initially there will be convection between fluid one and channel wall then conduction between opposite sides of channels walls and at last convection between channel and fluid 2. The total amount of heat transfer by the spiral heat exchanger is the overall heat transfer coefficient of the spiral heat exchanger. Overall heat transfer coefficient can be calculated as below (Metta, Konijeti et al. 2018)
1/U=1/h + x/k + 1/(h )
U = overall heat transfer coefficient
h = convective heat transfer coefficient
K is conductive heat transfer coefficient

 Finite Element Analysis Spiral Heat Exchanger
A lot of work has been done on the use and performance of spiral heat exchanger. The major impact on the performance of fluid inside the heat exchanger is of mass flow rate of fluid, so an optimized mass flow rate is needed for the optimized performance of heat exchanger. More studies has been carried out on the drop of pressure of fluid inside the spiral heat exchanger such as the thermal performance and drop of pressure in spiral heat exchanger (Metta, Konijeti, 2018). First Spiral heat exchanger was designed by (S.M Tandle 2008). the idea was to recover waste heat. Work concluded out that the flow in the spiral tubes depends upon following factors
Centrifugal force
Buoyancy force
Frictional  force
One of the researches was done on performance analysis of spiral tube heat exchanger used in oil extraction system. They mostly focused on the advantages of spiral heat exchanger over shell and tube heat exchanger and the effectiveness of spiral tube heat exchanger in the long run. The result of this study shows that for the same mass flow rate spiral tube heat exchanger is more effective than that of the shell and tube heat exchanger. 
They also focused on the effect of Reynolds number on heat transfer coefficient and the relation between Reynolds number and Nusselt number. They found out that the heat transfer coefficient increases with increasing Reynolds number and same is the case for Nusselt number 
The temperature and behaviour of the flow and found out that 
     1. The flow of heat exchanger is composed of two things
a. main central flow
b. thin boundary layer 
The thickness of the boundary layer depends upon the velocity of the fluid.

J. D. Seader (1972) studied the effect of viscous flow inside the spiral heat exchanger. He performed analysis by hand while keeping some parameters constant i.e the peripheral temperature. Result of his studies showed that the Nusselt number remains the same no matter what is the curvature ratio and no matter what is the Prandtl number .further researches has been carried out to find the heat transfer coefficient for different fluids such as benzene.
One major study was done to find the heat transfer characteristics and the performance of a spiral coil heat exchanger such as 
Mass flow rate
Inlet and outlet temperatures

Air and water had been used as running fluids. This take a look at used the famous mathematical version I.E Newton– Raphson iterative technique to determine the above characteristics for spiral tube warmth exchanger. They located that enthalpy, effectiveness and the humidity effectiveness reduced with increasing air mass drift charge for a given inlet-water temperature. The growth within the outlet enthalpy and outlet humidity ratio of air changed into large than the ones of the enthalpy of saturated air and humidity ratio of saturated air. Therefore, the enthalpy effectiveness and humidity effectiveness generally tend to lower with increasing air mass flow fee. They also determined that the effect of inlet-air temperature on the tube floor temperature. At a specific inlet-air temperature, the tube floor temperature generally will increase with increasing air mass flow fee; however, the increase of the tube floor temperature at better inlet-air temperatures was better than at decrease ones for the same range of air mass go with the flow charges. They observed that at a specific air mass drift charge, the tube floor temperature decreases as water mass flow increases. Finally the consequences acquired from the developed version are confirmed by evaluating with the measured information. On complete take a look at on warmness switch co-efficient and effectiveness for water the use of spiral coil warmness exchanger
Garcia et al proposed a numerical method for rating thermal performance in spiral plate heat exchangers. J. F. Devois (1995) performed a numerical 2D modelling of spiral plate heat exchanger. Performed both experimental and numerical analysis and derived an empirical equation for Nusselt number. M.D. Kathir Kaman (2017) performed analysis on SHE for cooling applications.
The heat recovered by using spiral heat exchanger can be utilized in purpose of heating, processing of paper and in electric power regeneration. Many different companies and researchers are thinking where to use the heat recovered from the exhaust of engine. . According to study of exhaust gases of diesel engine can be utilized for turbocharging, heating to cabin air of the power plant and generate electrical power for cabin axillary devices and exhaust fans .but the important concern in recovering heat from diesel engine is the amount of carbon content.
According to the look at of, backpressure in exhaust system is directly affect to the overall performance and overall performance of the engine. Design of exhaust device and after remedy devices are the critical factor to keep in mind at some point of their software. Moreover, before emitting the exhaust gases into environment, those gases are handled via emission control gadgets to treat harmful elements sitting in exhaust gases. Furthermore, finish from his research that diesel particulate clear out is a tool that may manage and clear out the carbon contents from exhaust gases and backpressure display tool helps to govern the returned stress.
Heat exchanger is one of the best equipment to recover heat from the exhaust gasses. But due to some limitations which are area toward exhaust gasses and content of carbon it is used for heat recovery .he also concluded from his study that shell and tube heat exchanger is the to recover heat. Tube carries water and shell carries the exhaust gases the heat is transferred from exhaust gasses to water and from this heat is recovered. However, this setup increases the backpressure within the system that limits its use with heavy duty diesel engine where higher engine efficiency is the basic requirement (Metta, Konijeti et al. 2018). 
Approximately ten to fifteen percent heat can be recovered by using shell and tube heat exchanger. Researchers also studied about different technologies that can be used with the spiral heat exchanger to recover heat from IC engines. Moreover, a research was conducted for the technologies, design, method and type of heat exchanger that can be implementing to recover heat from exhaust of compression engine.
Design of Spiral Heat Exchanger:
Heat exchanger is designed in two steps
Fluid and Thermal Design:
Thermal design is the main part of design because this calculates require heat transfer surface. In most cases overall heat transfer coefficient is calculated because it gives relation between heat transfers to the temperature difference of the system. All resistance parameters are included in heat transfer coefficient which is fouling resistance, cold fluid film, thickness of wall and hot film of fluid (Metta, Konijeti, 2018).
Mechanical Design:
 The purpose this step is to make heat exchanger with such material that it can bear loads in working condition. Spiral shape is made by rolling two parallel sheets around a central bar. Welding is done to seal the free edges. Studs are used to keep spacing between metal sheets. Size of the stud may vary. Hence, with respect to the mass flow rate, different distances between the sheets can be chosen during the design period. In each channel, hot or cold fluid path, secondary flows are developed that lead to better mixing and therefore heat transfer rate is increased and fouling is decreased.
Figure (a) Hot and Cold Channels (b) Completed Heat Exchanger (Jamshid Khorshidi 2016)

Thermal analysis of spiral heat exchanger:
Thermal analysis is done to know the behaviour of exchange of heat between two mediums. Analysis results can be used to improve performance of heat exchanger. 
Spiral heat exchanger is modelled using ANSYS design modeller. Inlets, outlets, walls and interfaces are defined inside the modeller. Bullen command is used to create a fluid region. Geometry is then imported in mesher for meshing. Meshing plays a important role in analysis increasing number of elements will increase accuracy of result. Uniform coarse mesh is used. Model is then imported to fluent for thermal analysis. 

Figure 3(a) 3d model of SHE (b) Model used for Analysis (Jamshid Khorshidi 2016)

To show finite number of elements on geometry meshing is performed. More number of elements means high accuracy and vice versa. Hexahedral mesh is performed on the geometry. The 3D meshed model is shown in Figure
Figure 4: Meshing of Geometry (Jamshid Khorshidi 2016)

Boundary Condition: It is difficult to define parameters at inlet and outlet that occurs in reality.at inlet velocity is defined as boundary condition. Velocity inlet boundary conditions are used to define the flow velocity, along with other relevant scalar properties of the flow, at the flow inlet. At outlet pressure is defined as boundary condition. The K-model is employed for turbulence modelling in the simulation of flows. Simulations are executed the usage of Nitrobenzene at 800C as hot fluid and water at 200C as cold fluid. For a warm and cold fluid drift fees, the temperatures and the warmth transfer coefficients at the outlet of hot and bloodless fluids are referred to. The final temperature contour effects after the simulations are given within the Figures 3. It can be visible that the cold fluid enters into the outer edge of the SPHE with a temperature of 200C and leaves on the critical centre of the warmth exchanger. On the opposite hand, the hot fluid enters into the important centre of the heat exchanger with a temperature of 800C and leaves it at the outer edge of the warmth exchanger. The variation of temperature of hot fluid and cold fluid along the length of the central line of the spiral passage for different hot and cold fluid flow rates is as shown in Figure