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You require choosing or designing a heat exchanger for a waste heat recovery system in cars.

Heat Exchanger selection/design

Imagine you are the part of a product development team which is developing a novel waste heat recovery system to make cars more efficient. One feature of the system is to transfers wasted exhaust heat to engine oil via a heat exchanger to reduce the friction of the oil, particularly during warm up, but also during steady state operation at engine loads below wide open throttle. Your task is to choose and source an existing heat exchanger that would be suitable for this application. Even though most driving patterns in a car are highly dynamic you are given most operating conditions as average over the legal fuel consumption drive cycle. In the assignment you require to justify the decisions based on you engineering judgement and by applying the concepts you learned.

The priorities for new product are as follows:

1. Safe operation under all possible operating conditions at ambient temperatures between -30 °C and +50 °C

2. Maximum efficiency of the heat exchanger to be able to realise the highest possible fuel economy improvement

3. Durability of 150,000km (legal requirement for emission relevant components)

4. Low weight to minimise the negative impact of additional weight towards fuel economy

5. Small external volume to make it easy to package it in the packed and tight engine compartment

6. Low cost

7. Low external temperatures to avoid deterioration, aging  or long term damage of nearby plastic components

8. Long term resistance against fouling

Operating conditions

The following operating conditions must be considered in your selection or design process:

• The test condition is the New European Drive Cycle (NEDC), the duration is 1180 sec

• The exhaust gas consists of corrosive combustion products. The average water content is about 10%

• The average exhaust temperature into the heat exchanger is 434 °C during the NEDC

• The maximum exhaust gas temperature is 625 °C during the NEDC

• The ambient air temperature is 24 °C during the NEDC, this doesn’t change over the test cycle

• The start temperature of all engine components inclusive the exhaust gas heat exchanger and the engine oil is also 24  °C

• The average flow rate of the exhaust gas is 6.9 g/s

• The average oil flow rate is 10 l/min

• The average oil temperature is  60 °C

• The average vehicle speed is 35km/h, the wind speed is simulated proportionally to the vehicle speed by a vehicle cooling fan

• The average exhaust heat flow during the NEDC is 4kW

• The target exhaust gas temperature after the heat exchanger is to be below 90 °C during most parts of the NEDC to be able to utilise the condensation enthalpy of the water as much as possible

Deliverables

The following deliverables are needed as the outcome of your development work, independent if you chosen an existing heat exchanger or if you designed your own. If some numerical values that may be required for your calculations are not given in the assignment, you need to estimate them and you need to justify these estimations. Whenever you make assumptions you need to clearly articulate the assumptions, cite the source of your assumptions and add a comment about the potential effect of the uncertainty of your assumptions.

• Material specification that will be used and material thickness

• Average efficiency or effectiveness

• Overall weight

• Outside dimensions

• Package volume

• Maximum average outside surface temperature during NEDC

• Maximum average inside surface temperature during NEDC

• Oil volume in the heat exchanger

• Heat required to warm up the heat exchanger (a lump sum analysis is sufficient)

• Average heat rejected to the ambient

• Effective heat transfer surface

• Layout with definition of inlet and outlet for oil and exhaust

• Sketch of the cross section

• Cost (single unit prototype and  if mass produced at 100,000 units per year)

2200-2500 words limit and at least 10 reference.

Mechanical Engineering, Engineering

  • Category:- Mechanical Engineering
  • Reference No.:- M91359

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