Design Flow

Design Flow

The design of the system is done in a specific chronological order.

  • The heat load was calculated using the manufacturer dynamometer data and IPG software.
  • Selection of sizes, radiator size and fan, was done according to the data.
  • CFD analysis was done to obtain the airflow velocities at the inlet face of the radiator and the pressure drop across the radiator.
  • Wind tunnel test is also performed to obtain results for CFD validation. A better idea of the behaviour of air through the core could be studied.
  • The components are arranged in a manner to fulfil all the requirements and constraints of individual components.
  • The entire system design was optimised according to the values from various iterations and optimal design was finalised.
  • Once a system is designed and the vehicle is ready, various tests can be run to obtain the test data to validate our design.

Arrangement of components in the order of flow

The cooling system consists of the Motor, Motor Controller, a Heat exchanger (Radiator) and circulating pump. A specific order for the flow of coolant from one component to the other throughout the system is determined. This flow is decided to depend upon the various operating conditions and the allowable temperature limits of the components.

The relative arrangement of the components was done in a specified way due to the following reasons:

  • The motor controller, as specified by the manufacturer requires a coolant inlet temperature of 45 ֯C. Above which the motor controller would start to decrease its efficiency of running. The motor on the other hand could run efficiently at below 60 ֯C.
  • Due to this reason, the motor controller was placed before the motor, to allow partially heated coolant into the motor and cooled coolant from the radiator into the controller
  • The pump was placed after the motor. This is because the maximum pressure allowable inside the motor coolant jacket was 2 bars.
  • A pressure drop would occur through the radiator and the motor controller. The pressure differential as specified by the controller manufacturer was 1.3 bars which were essential to overcome at a flow rate of 10 lpm.

The radiator is placed last in the sequence so that the coolant with the highest temperature is cooled in the radiator.

Pump Selection

To determine a pump, it was first necessary to determine the system’s required water flow rate and pressure. To maximize heat transfer, the maximum permitted flow rate was selected. Determining the required flow rate was as simple as observing the specifications of the motor and motor controller. The maximum flow rate permitted (recommended by the manufacturer) by the motor controller was smaller than the maximum flow rate permitted by the motor. Therefore, the maximum flow rate permitted by the motor controller, 12 LPM, was selected as the flow rate of the system. To determine the pressure required, it was necessary to determine the pressure drop due to each component as well as the pressure loss through the coolant lines of the system.

 

To determine the pressure drop due to the components of the system, the following equation was used:

∆???? = ????????????2

where ∆???? is the pressure drop across a component, Qw is the flow rate through a component, and k is the loss coefficient of a component. Moreover, the total pressure drop across the system can be written as the following:

∆???? = (???????????????? + ???????????? + ????????????)???????? 2

where kMTR is the loss coefficient for the motor, kMC is the loss coefficient for the motor controller, and kHX is the loss coefficient for the radiator.

The pressures provided are absolute. The inlet pressure for a flow rate of 12 LPM was found via interpolation and was used to solve for the pressure drop across the motor, assuming the exit pressure is atmospheric (1 bar).

Using this pressure drop, the loss coefficient of the motor was calculated as follows.

The pressure drops of the motor controller and the radiator need to be calculated physically by testing.

Once the total system pressure loss is calculated, we need to find a pump which can deliver water at 12 lpm at this minimum pressure loss because It will constantly have to overcome this pressure loss.

A system resistance curve using the data calculated above can be calculated for various flow rates and a plot pressure drop vs flow rate can be made.

The manufacturer can provide a performance curve for the selected pump.

This can be used to select the pump using the following steps –

  • Scale both the graphs on a common scale for both the axes.
  • Plot the graphs and mark the intersection point.
  • This is the best-case scenario is to use the pump
  • If this value doesn’t match with your desired flow rate, check the pressure that the pump can provide at that particular pump. This value should be above the system resistance that the pump will have to overcome.

Hence use these steps to select the pump for the system.

Coolant Line Diameter Selection

It was necessary to decide an appropriate coolant line internal diameter for the system. This was a critical task due to the mix of inlet and outlet sizes throughout the system. Unfortunately, the pump inlet and outlet diameters are designed for a 1-inch inner diameter hose while the motor and motor controller inlets and outlets are designed for a 3/8 inch inner diameter hose. If a 1 inch ID is used, many unusual or custom fittings must be used to fit the hose to the motor and motor controller. However, a 3/8 inch ID hose has a significant pressure loss due to friction. Therefore, the pressure loss through the hose was determined for numerous inner diameter sizes. Based on the vehicle geometry, a hose length of 4 feet was used for calculations. The first step in this process was to determine the Reynolds number

Note that D is the inner diameter of the hose, A is the area of the hose, and ???? is the kinematic viscosity of the water. After determining the Reynolds number, the Moody friction factor, f, was determined using the following equation:

where ∈ is the absolute roughness of the rubber tube and is equal to 0.0016 millimetres. The pressure loss due to friction in the hose could then be calculated using the following equation:

where L is the length of the hose and all other variables are consistent with previous definitions. Equations 46, 47, and 48 were used to calculate the pressure loss for hoses with inner diameters of 3/8 inch, 1/2 inch, and 5/8 inch.

Once you find the pressure loss in pipes, add this to the total system loss.

Total pressure can be found from the pump performance curve and the total loss can be found from the calculated values. Hence it should be made sure that the pressure that the pump can provide should always be greater than the loss to avoid cavitation at the entry side of the pump.

Fan Selection

Like a pump, a fan can pull or push air so that it travels at a flow rate specific to a pressure drop. That pressure drop is a product of the system design, and the system pressure drop varies with the velocity of the flow. As a result, each fan-system combination has an equilibrium point at which it will operate.

The figure also shows a stall region where the flow separates from the fan blades, which create vortices. These vortices result in backpressure, which is reflected in the figure. An engineer will be able to determine the static pressure drop across the radiator so that they can choose a fan, based on the manufacturer’s fan curve if they decide to use one. Initially, when a flow bench was going to be built to facilitate testing, an estimate the system pressure loss (losses due to ducting, duct components, and radiator core) was needed to size the fan that would be used on the flow bench. We were provided with a sample data point which indicated the static pressure drop across a radiator core at a given speed: 50 ft/s and 57 psi pressure drop. With this data, the loss coefficient of a typical radiator core could be estimated.

In estimating the major losses in the ducting and minor losses in the duct components at the maximum speed at which the flow bench would be operated, we could choose a fan. The fan should have been able to operate such that air would flow at the necessary maximum speed and overcome the calculated system backpressure. When we decided to conduct testing on the wind tunnel in the Thermal Science Lab, we assumed that the fan on the wind tunnel would be adequate. Typical fan-system curve 17 velocities of up to 200 mph with no added loss elements in the ducting. We concluded that it would be adequate to get our desired airspeeds.

Shroud Design

Factors Influencing Shroud Design Certain factors govern the design of the fan shroud. These factors are as follows:

  • Standoff distance of fan from the radiator core
  • Inner wall curvature
  • Additional spline requirement for a robust design
  • Manufacturability
  • Cost of production

The desired standoff distance is a tradeoff between pressure, flow velocity and space constraints of the manufacturer. A well-designed inner wall curvature increases the flow characteristics within the shroud. Splines are required to provide additional structural strength to the shroud since the fan is mounted on it. Availability of raw materials and process used to manufacture the shroud also plays a vital role in reducing the overall cost of production. The radiator shroud used in most vehicles is a standard device that is merely used to hold the fan in place. However, there are a lot of changes which can help improve the thermal efficiency of the vehicle. The proposed shroud design seals the space between the radiator and the fan forcing all air to pass through the radiator core. The shroud does not allow cross mixing of heated air in the engine compartment with the incoming cool air hence increasing radiator efficiency. Uniform heat rejection from the surface is developed since the shroud balances the pressure gradient across the face of the radiator. A lightweight and compact design can be easily incorporated for commercial use. Hence, the present work focuses to develop an efficient thermal management system for an automobile.

Design of a Shroud in the Radiator Assembly The automotive cooling system comprises of an aluminium heat exchanger, fan shroud and a cooling fan. A custom made aluminium radiator of specific dimensions has been used for this study. This is a single pass, down-flow type radiator. The radiator is a type of heat exchanger which consists of thin tubes through which the hot coolant passes. The shroud was designed as shown in Figure 1 by keeping the specifications of SPAL High-Performance cooling fan of an 8-inch diameter which has straight profile blade geometry. The shroud is attached to the radiator using mechanical locking and fasteners are used to mount the fan to the shroud. The fan motor is of considerable weight and hence the shroud should have structural fortifications to sustain the load. Loctite 95 G Tube Rtv Silicone Gasket Maker Si 587 sealant is also used to prevent leakage through gaps.

Reference

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