1. Modelling of Frost Formation and Growth on Microstuctured Surface
11th International Conference on Mechanical Engineering (18-20 December, 2015) Dhaka, Bangladesh
Modelling of Frost Formation and Growth on Microstuctured Surface
Md. Ali Muntaha Md. Mushfique Haider Md. Ashiqur Rahman
Department of Mechanical Engineering,
Bangladesh University of Engineering and Technology (BUET), Dhaka-1000
Bangladesh University of Engineering and Technology, BUET
Paper ID: 542

2. Background Frost is a deposit of small white ice crystals
Paper ID: 542
Background
Frost is a deposit of small white ice crystals
Formed on the various simple and complex geometrical surfaces when the temperature falls below freezing
Application: Refrigeration, Cryogenics, Heat Pump, Air Conditioning etc.
Effect of frost formation
Nawaf F. (2006)

3. Background and Motivation
Paper ID: 542
Background and Motivation
The Modelling of frost formation is essential as because performance of a heat exchanger under frosting condition is characterized by frost properties.
Modelling enables to predict the frost properties
Frost properties are known to be affected by
temperature and moisture content of air
cold plate temperature ,
air velocity and
surface wettability
Hoke et al. (2004)

4. Motivation
Paper ID: 542
The ability of a liquid to maintain contact with a solid surface is the surface wettability
Modelling incorporating wettability effect on flat surfaces
Wettability effect on micro-structured surfaces
Importance of micro-structure surface
No study could be located in available literature
Rahman and Jacobi (2013,2015)
Micro-groove
Flat

5. Paper ID: 542
Research Objective
To model the frost properties for various flat and micro-grooved metal surfaces having different geometries
To incorporate the effects of wettability on frost formation and frost properties
Validate the findings with experimental results [Rahman and Jacobi (2013)]
Comparison of frost properties on flat and micro-grooved metal surfaces

6. Paper ID: 542
Numerical Method
The total mass flux transferred from the moist air to the frost surface can be subdivided into two parts
The equation can be written as
The frost growth rate equation is obtained
The heat transfer through the frost surface by conduction

7. Numerical Method Boundary Condition at t=0
Paper ID: 542
Boundary Condition at t=0
The temperature at the frost surface corresponding to x = 𝑥 𝑠 can be obtained from
An empirical correlation by O’Neal and Tree(1985)
Different initial thickness and density for different geometries

8. Results Paper ID: 542 Error 14% for DG = 122 µm Error 17% for
Wp = 80 µm
Initial density = 75 kg/m3
Initial density = 85 kg/m3
Wp = 110 µm
WG = 130 µm
DG = 67 µm
WG = 130 µm

9. Results Paper ID: 542 Error 7% for Wp = 80 µm
Initial Thickness = 2.65mm
Error 7% for DG = 122 µm
Initial Thickness = 2.2mm
Wp = 110 µm
WG = 130 µm
DG = 67 µm
WG = 130 µm

10. Paper ID: 542
Results
Tao & Jia (Critical review,2004)
Operating conditions: Ta = 22±2°C, Tw = -18°C, t = 0 to 225 min, RH = 30-70%
Operating conditions: Ta = 25°C, Tw = -15°C, t=0 to 120 min, W = kga/kgw.

11. Frost density is predicted quite accurately with respect to time
Paper ID: 542
Results
Wp = 110 µm
DG = 67 µm
Frost density is predicted quite accurately with respect to time
Thermal Conductivity is predicted quite accurately with respect to density

12. Results Initial thickness and density calculation by image analysis
Paper ID: 542
Results
Droplet Distribution
Label
Area
Perim. 1
0.005
0.331 2
0.032
2.551 3
0.092
4.182 4
0.427
7.06 5
0.009
0.562 6
0.02
0.951 7
1.256 8
0.147
8.172 9
0.04
3.082 10
0.008
0.793 11
0.01
0.698 12
0.034
1.778 13
0.801
43.142 14
0.671 15
0.815 16
0.015
1.389 17 18
Mean
0.099
4.603
Initial thickness and density calculation by image analysis
Volume calculation
Frost mass
Density Calculation
Amne El Cheikh (2013)
Droplet Distribution (Top View)
Image Analysis

13. Summary A model incorporating the wettability effect is developed
Paper ID: 542
Summary
A model incorporating the wettability effect is developed
Incorporating initial thickness and density values from experimental results, a model is developed for different flat and micro-grooved geometries for a specific operating condition.
Our model can satisfactorily predict the frost properties
Maximum error of 7% for frost thickness and of 17% for frost density
We are continuing our work for obtaining the initial frost thickness and density values directly form droplet image analysis

14. THANK YOU!
Bangladesh University of Engineering and Technology, BUET