Analysis of heat & air recirculation inside a refrigerator

Thermofluids

PRESENTATION ON

By Dibyajyoti Laha Anglia Ruskin SID: 1227201

A Bit of introduction !

China United States

Japan India Brazil France0

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Statistics as per 2004 Domestic refrigerator use (in billions)

• On growing demand for refrigeration

• Rising growth of food production storage

• Fast paced life.

• On growing demand from the industrialized countries.

• Demand for better cooling and efficiency.

• Efficiency rating by EU laws.

Types of refrigerator

Static

Brewed

No-Frost

Basic forms of heat transfer

Methods of heat transfer

• Conduction – transfer of heat through contact.

• Convection – transfer of heat through air / fluid.

• Radiation – transfer of heat by emission.

Heat transfer in a refrigerator

Closed cavity heat transfer

Use of Natural Convection in refrigerator heat transfer:

Heat transport without external force. Generally heat received by the fluid becomes less dense and is replaced by the surrounding fluid.

Air circulation

Types of air flow:

• Laminar flow - Stream line flow

• Turbulent flow - flow having irregular fluctuations

General law: Air being hot moves upwards while colder replace it.

Air circulation in refrigerator • Cold air from evaporator flows downwards

• Warm air near the doors and other sides flow upwards.

• Flow is governed by the natural convections of heat transfer.

Evaporator Side (Cold) Door / side warm

2D presentation of air flow inside refrigerator

Using ANSYS CFD• Powerful tool for analysing fluid flow. • Understand the behaviour of the fluid domain • Determine virtually the heat transform, air velocity &

recirculation patterns.

Analysis & Fluent requirements

• 2D geometry of the refrigerator’s cavity/ evaporator.

• Meshing element size with minimum size – 2mm maximum size – 4mm

• Computing solver 4 processors with double precision

• Energy equation, 2 equation model K-omega

• Name geometry values • Running iterations for accuracy.

Governing Equations & Methods

• Navier Stokes Equation

• Equation of continuity

• Raleigh’s number

• Numerical Discretization Method

• Finite Volume Method

Results form CFD• Top wall cold• Bottom wall warm • Adiabatic symmetrical walls

Top cold region around 255.15oKGradually temp. starts increasing 263. 15oKTill it reach the warm end its 273oK

Velocity with blue around 0 m/s While greenish in 1.47e-07 Red region of higher air flow.

• Top warm wall• Bottom cold wall• Adiabatic symmetrical walls

• Side Cold wall • Door warm side• Adiabatic symmetrical walls

Use of fan (inside refrigerator)• Top cold wall, bottom warm wall• Adiabatic symmetrical walls • Fan on left

• Bottom cold wall, top warm wall• Adiabatic symmetrical walls • Fan on left

Application of fan • Faster cooling • More efficient• Aids in defrost process• Modern efficient process and greater efficiency

Radiation heat transfer -

• Radiation improves heat transfer • Clearer picture of temperature difference• Experimental Analysis (O Laguerre) shows

without radiation maximum temperature can rise up to 15 degree C

Conclusion

Best Practice:

• Use of bottom cold wall and top warm wall

• Application of internal fan for better fluid flow

• CFD an influential tool to study fluid flow (heat transfer as temperature & Air flow as velocity, turbulence.

Comes next is the material selection ……………