Project

Flow Simulation of a Counter Flow Heat Exchanger

RRohit Beniwal20 Jun 2020

1.AIM :

TO MODEL AND PERFORM THE CFD ANALYSIS OF A DOUBLE PIPE COUNTER FLOW HEAT EXCHANGER

2.OBJECTIVE :

To model a double pipe counter flow heat exchanger using Solidworks.
To perform CFD analysis of the double pipe counter flow heat exchanger using Solidworks flow simulation.
To find the temperature of both the working fluid after reaching equilibrium.
To calculate LMTD of heat exchanger.

3.INTRODUCTION :

A heat exchanger is a system used to transfer heat between two or more fluids. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact.

They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment.

Types of Heat exchanger

1) Based on Design characterstics

Different variants of heat exchangers based on design are

Shell and tube heat exchangers are constructed of a single tube or series of parallel tubes enclosed within a sealed, cylindrical pressure vessel (shell). The design of these devices is such that one fluid flows through the smaller tubes, and the other fluid flows around their outside and between them within the sealed shell.

Double pipe heat exchangers employ the simplest heat exchanger design and configuration which consists of two or more concentric cylindrical tubes (one larger tube and one or more smaller tubes). Here one fluid flows through the smaller tube, and the other fluid flows around the smaller tube within the larger tube.

Plate heat exchangers are constructed of several thin, corrugated plates bundled together. Each pair of plates creates a channel through which one fluid can flow, and the pairs are stacked and attached via bolting, brazing, or welding such that a second passage is created between pairs through which the other fluid can flow.

Condensers, evaporators, and boilers are heat exchangers which employ a two-phase heat transfer mechanism. They are two-phase heat exchangers in which one or more fluids undergo a phase change during the heat transfer process, either changing from a liquid to a gas or a gas to a liquid.

2) Based on Flow configuration

Counterflow heat exchanger has the hot fluid entering at one end of the heat exchanger flow path and the cold fluid entering at the other end of the flow path.

Parallel flow heat exchanger has both fluids enter at one end and exit at the other end.

Crossflow heat exchangers has fluids flowing perpendicularl to one another.

Hybrid flow heat exchangers exhibit some combination of the characteristics of the previously mentioned flow configurations. For example, heat exchanger designs can employ multiple flow passes and arrangements (e.g., both counter flow and crossflow arrangements) within a single heat exchanger.

Flow simulation through a Heat exchanger can be performed through computational fluid dynamics (CFD), a powerful and convinient tool to perform CFD is Solidworks flow simulation.

Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate the free stream flow of the fluid, and the interaction of the fluid with surfaces defined by boundary conditions.

In simple words to perform CFD following steps are performed in order:-

A computational model is made using computer aided designing and the domain of fluid is extracted.
Further the domain is divided into discrete cells called mesh.
Next the Boundary conditions are defined, which is specifying the fluid behaviour and properties at all bounding surfaces of the fluid domain.
Next the calculations are performed and the equations are solved iteratively till the convergence is acheived.
Finally the analysis and visualization of the calculations performed can be seen.

Here in this work the modelling and CFD analysis is performed for Counter flow double pipe heat exchanger having multiple pass.

The cut section of model can be seen below.

The fluid 1 selected for the study is water (entering at 10°C) and fluid 2 selected is ethanol (entering at 78°C) and the outlet temperature of both the fluid is found out after equilibrium has reached. Further calculate the Logarithmic mean temperature difference (LMTD).

The log mean temperature difference (LMTD) is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger. The larger the LMTD, the more heat is transferred.

4.PROCEDURE :

1) At first the model of a double pipe heat exchanger is created using Solidworks.

2)Then inlet and ourlet ends of both the tubes/pipes are closed with lids using create lids option present in ribbon.

3)Using the wizard option in Soildworks flow simulation a new project is created. The analysis type selected is 'Internal' and 'Heat conduction in solids' analysis is chosen.

4)Water and ethanol are chosen as the working fluids.

5)Copper and Brass solids are chosen, but initially copper is selected and brass will be taken later.

6)For initial conditions, in thermodynamic parameters temperature is entered as 30°C and concentration of Ethanol is entered as 1 and of water is entered as 0. Also the inital solid temperature is entered as 30°C.

7)Using the fluid subdomain option the inner tube/pipe is selected for the fluid ethanol to pass, the temperature of entering ethanol is selected as 78°C and the outer tube/pipe is selected for the fluid water to pass, the temperature of entering water is selected as 10°C.

8)Using the Insert solid material option, the material for the inside tube/pipe is selected as copper and the material for the outside tube/pipe is selected as brass.

9)At the inlet of the inner tube/pipe the boundary condition applied is temperature of 78°C & mass flow rate of 0.001Kg/s for fluid ethanol and at the inlet of the outer pipe the boundary condition applied is temperature of 10°C & mass flow rate of 0.001Kg/s for fluid water.

At oulet of both the tube/pipe the boundary consition is selected as Environmental pressure.

At the wall of the outer tube/pipe the boundary condition selected is Real wall with Heat transfer coefficient as 60 W/(m²K) and temperature as 30°C.

10)Surface goals are added at the outlet of both the tubes/pipes to find the average temperature of the fluid after heat exchange.

11)A basic mesh is defined with mesh grid size setting as 4.

12)Finally the simulation is run.

5.RESULTS :

After 254 iterations the soultion got converged and the equilibrium temperature of both the fluids after heat exchange was found.

The 2 graphs below shows the decrease in temperature of ethanol and increase in temperature of water and both finally reaching the equilibrium.

The temperature of ethanol and fluid coming from outlet after reaching equilibrium:-

FLUID	ETHANOL	WATER
TEMPERATURE AT INLET (°C)	78	10
TEMPERATURE AT OUTLET (°C)	46.44	49.38

Using these values to calculate Logarithmic mean temperature difference

Where, Hot fluid - Ethanol

Cold fluid - Water

`T_(hi) = 78°C`

`T_(he) = 46.44°C`

`T_(ci) = 10°C`

`T_(ce) = 49.38°C`

`"LMTD "= (DeltaT_i- DeltaT_e)/(ln( (DeltaT_i)/( DeltaT_e)))`

`"LMTD "= ((78-49.38)- (46.44-10))/(ln( (78-49.38)/(46.44-10)))`

`LMTD=32.37°C`

Cut plot for temperature contour (front view)

Cut plot for temperature contour (right view)

Flow trajectories showing temperature variation

The decrease in temperature of the ethanol (in inner tube/pipe) and increase in temperature of water (in outer tube/pipe) can be seen from the temperature contour and flow trajectories.

The animation of flow trajectories showing temperature variation is given below

6.CONCLUSIONS :

The simulation of double pipe counter flow heat exchanger was completed successfully using Solidworks flow simulation.
The final temperature of both the fluids obtained was

TEMPERATURE OF ETHANOL AT INLET = 78 °C

TEMPERATURE OF ETHANOL AT OUTLET = 46.44 °C

TEMPERATURE OF WATER AT INLET = 10 °C

TEMPERATURE OF WATER AT OUTLET = 49.38 °C