Heat Transfer in Radiators

Jignesh Gandhi

Heat transfer in Radiators

n Why Cool Engines?n Engine Cooling Modelsn Engine Cooling Requirementsn Heat Rejection Curvesn How cooling Levels are reported?n Why add Ethylene Glycol?n Radiator Performance Curves

Why Cool Engines?

n To analyse energy transfer in radiators, we must ask first “Why should we cool Engines?”

n Understand this by simple model on next slide.

n Based on laws of Thermodynamics.n First Law – Energy can neither be created

nor destroyed but can only change from one form to another.

n Entropy of an isolated system can never decrease.

Energy Balancen Input Energy of fuel converted to

output mechanical rotational energy. But with only max 33% efficiency.

n Out of 100 %1/3 energy is mechanical energy.1/3 energy is lost to exhaust gases.1/3 energy converted to waste

heat.

System Componentsn Radiator, Water Pump, Fan, Thermostat

all form a cooling system.n Each component has design

characteristics and limits and hence Radiator design is a collaborative effort of all these subsystems.

n We want to further extract energy from exhaust gases and hence the business of CAC.

n We also want to cool the oil and hence the business of oil cooler.

Engine Cooling Requirements

Typical Heat Rejection curves

n Curves developed by the manufacturers.n Plotted against engine speed as shown.n High Power Condition also called rated

power. Generally this is the maximum power developed by engine at given conditions.

n High torque is also a critical condition. n Radiator which meets high power

condition may or may not meet high torque requirements of the engine.

Heat Balance n In a system, heat of engine is brought to

radiator by the coolant. This is transferred to tubes and through fins to the cooling air.

n Hence in simplistic terms, Heat given by water = M Cp (Tw in – Tw out)Heat Taken by air = U * A * LMTD

M- Mass of water flowing in kgCp – Specific heat at Const. Pr Kcal/kg/0CTwin – Temp of water at inlet to radiatorTwout- Temp of water at outlet to radiatorU – Overall heat transfer coefficient KCal/min/m2- 0CA – Core area (Heat transfer area)LMTD – Log mean temp Difference.between heat

transfer fluids. 0C

Coolant Flow Datan As flow rate changes, the pressure

drop across the tubes in waterside will also changes.

n Hence for accurate design full data on pump characteristics is needed.

n In reality, we only get data about flow rate at max power condition. This is a big problem.

Fan Curvesn How much air fan will deliver will change

based on fan characteristics.n Data generated by fan manufacturers.n Data different for Truck radiators and car

radiators.n Truck fans are engine driven and hence

variable speed.n Car fans are generally electrical.n Constraints in actual design to airflow due

to front hood, other parts etc also very important.

Coolantn Mixture of Ethylene Glycol and Water.n 50% most common.n Increases boiling point of water from

1000Cto 120 0C when used with 7 PSI pressure

cap.Also gives freezing protection to –37 0C .This allows us to design and operate

radiators over a larger temperature range.

Radiator Performance Curves

n These curves are plotted for the given radiator for different air flow ratings and related heat rejection characteristics.

n Also plotted are the air pressure drop curves for the same configuration.

n For increasing the heat transfer, we use the louvered fins or other fins which generate turbulence to improve the overall heat transfer coefficient.

What is the louver angle, how many fins per inch, how many tubes, how many rows, depth of core these are the issues which have to be decided in design to arrive at the configuration which achieves heat performance.

Conclusion

n Radiator heat performance design is dependant on many input conditions.

n Some are specified by the customer and some are assumed by the designer.

n Heat transfer in louvered fins is a complex subject on which lot of research is in progress.

n Many times designer works with insufficient data and hence design fails in some condition.

n Design assumes 100% process capability. But if there are problems in process, good design will also fail.

n Lot of experimentation, verification and validation is mandatory requirement.