2-DIMENSIONAL CFD SIMULATION
OF THE AIR FLOW INSIDE A LEMANG
OVEN
SUHAILA HUSSAIN1, ROSHALIZA HAMIDON2
School of Manufacturing Engineering University Malaysia Perlis, Kampus Ulu Pauh, Arau, Perlis.
http://www.unimap.edu.my
Abstract :
This paper presents the results obtained from a computational fluid dynamics simulation of the cooking process of a Malay delicacy called lemang inside a specially made oven. The normal way of cooking lemang is by putting it in open fire for more or less 2 hours. By using the lemang oven, the cooking time was reduced to about 1 hour and 20 minutes. A 2-dimesional CFD simulation was done to look at the hot air distribution inside the oven and how it affects the conditions inside the oven and the lemang during cooking. Results for velocity profile, as well as turbulence kinetic energy were obtained. It was found that the turbulence which carries with it energy was highest at the bottom of the lemang which would mean that that part of lemang would cook the fastest as it receives greater amount of energy compared to other parts of the lemang. This was why the lemang was overcooked at that part as evident from the experimental results. These results would be used to further improve on the existing lemang oven. .
Keywords: computational fluid dynamics, heat transfer, lemang, oven.
1. Introduction
Lemang is a Malay traditional food and is made of a mixture of glutinous rice and coconut milk placed inside a bamboo stalk. It is cooked by placing it near an open fire until it is fully cooked. This usually takes about 2 hours and it is required that the bamboo stalk be turned alternately every 10-15 minutes to ensure that all parts of the lemang is evenly cooked. The long time required as well as the laborious task of looking after the lemang warrants a more practical method to cook lemang. Due to this, a specially developed oven was fabricated in the hopes of eradicating these problems. It was found that the cooking time of lemang was able to be reduced to about 1 hour and 20 minutes when it was cooked in the oven. The quality of the lemang cooked inside the oven does not differ much from the ones traditional cooked in terms of taste and texture.
Computational Fluid Dynamics has been used by various researchers to simulate the processes involved in food products especially ovens. Therdthai et al used CFD to study the temperature distribution inside an industrial continous bread baking oven [1]. Saxena et al studied the contribution of each heat transfer modes inside a tandoor oven during the baking of roti [2]. Boulet et al, used CFD to model a bakery pilot oven in order to study all three heat transfer mechanisms combined with the effects of turbulence [3]. Hitesh et al used CFD to model a 3-dimensional, unsteady, natural convective flow-thermal field coupled with radiative heat transfer inside an electric oven [4]. Mostly, the researches here used CFD to either design a new oven or to improve on existing ovens so that a better quality of food is obtained.
The purpose of the CFD modeling and simulation presented in this paper is to understand the air flow profile inside the lemang oven and hence the heat distribution. Possible problems with the existing lemang oven arrangement and ways of improving it to obtain a better quality of lemang can be known.
2. Simulation Procedure
2.1. Oven Geometry
from the side and above. The lemang oven is 215 mm in length and width and 685 mm in height. There are 4 cylindrical trays of 530 mm in height wherein the lemang is cooked. Three heating elements are located on the sidewalls as shown in Figure 1. A fan is also located on the wall for the purpose of generating forced convection inside the oven.
Figure 1: Geometrical overview of lemang oven.
2.2. Modeling Assumptions
Unsteady state flow condition was used in the simulation. A 2-dimensional model was constructed with only a quarter of the real oven geometry taken into account. The computational time was able to be reduced but the main reason was that a 2-dimensional model was felt sufficient to obtain the desired objectives and to simulate the whole oven would be redundant. To account for the turbulence of the airflow inside the oven, the simple two-equation k-epsilon turbulence model was used. A segregated solver was used as again this would reduce any complications and computational time.
2.3. Boundary Conditions
For boundary conditions, the fan was set to be a velocity inlet as it is a source by which air enters the oven. The absolute velocity of fan was approximately 12.57 m/s according to the measurement taken using anemometer. Due to the available experimental data, the heating element was set as wall with temperature properties defined in the model together with the heat source generated from the heating element. The heat source generation rate was 1380 W/m3 based on the rated power of the oven. The rest used the wall and symmetry boundary conditions. The effect of conduction and convection was included in the wall boundary conditions but not radiation.
Initial conditions to the simulation were obtained from experiments carried before. The temperature readings were taken using thermocouples at certain positions in and outside the lemang and oven.
2.4. Modeling Equations
x yx xx x y x x x
g
x
P
y
x
y
v
v
x
v
t
(2) x yy xy y y y x yg
y
P
y
x
y
v
v
x
v
t
(3) Energy conservation: 2 2 2
2 2 2 2 2 x y y x y T x T k y T v x T t T
C x y x y
y x p
(4)As the problem also involves turbulence, the additional two transport equations for turbulent viscosity and length scales are also solved.
3. Result and Discussion
The simulation was completed with full convergence at the set time step of 4800 seconds corresponding to the time it takes for the lemang to cook in the experiment. At this point there was not much variation in the temperature inside the oven as the temperature was maintained at 150oC using a thermostat. This particular temperature is the temperature at which the lemang is estimated to fully cook based on the initial experiment.
3.1.Flow Profile
Based on Figure 2, the velocity of the hot air is higher near the bottom of the lemang. The velocity is higher there as it has the contribution of both forced convection from the fan on one side and natural convection from the heating element on the other side. Both opposing the paths of one and the other.
The velocity of air further up the lemang decreases possibly due to loss in energy as shown in Figure 3 and the effects of gravitational forces.
(a) (b)
Figure 3: Turbulence kinetic energy (a) side view (b) top view.
Based on the turbulence results, much energy is concentrated at the bottom of the lemang. This results in this part getting warmer faster than the other parts of the lemang. This can be seen from the experiment conducted whereby the bottom part of the lemang had a harder texture than the upper part (Refer to Figure 4). The upper part is still in its semi –solid state where as can be seen from Figure 3, not much energy reaches there. There was also not much heat concentrated in the area near the fan as it was possibly be convected away by the fan due to its close proximity. Figure 3(b) suggests that the sides of the lemang which are facing the middle section of the lemang oven will be cooked faster due to the high concentration of turbulence kinetic energy in this region. Furthermore, most of the energy seems to be concentrated at the bottom of the lemang near the heating element. So, there is an imbalance of heat distribution inside the oven. This is clearly shown in Figure 4.
Figure 4: The lemang after it is cooked inside the oven.
Another possibility is the strength of forced convection in terms of the speed of the fan. Increase in fan speed might be able to overcome the congestion of heat at the bottom of the lemang. This however, might well distribute the heat to one side of the lemang (the one near the heating element) but not the side which is near the fan.
Finally, based on the simulation results, having a large fan size does not necessarily mean that the flow of hot air can be spread through a large area. The size of the fan can be reduced but this would be relative to the arrangements of the oven decided.
The feasibility of these suggestions would require further simulations and study.
4. Conclusion
A 2-dimensional air flow simulation inside a lemang oven was conducted. The purpose of the simulation is to determine the conditions inside the oven during the cooking process of lemang in terms of heat distribution. Some interesting findings regarding the effects of the lemang oven arrangement on the air flow pattern were obtained. Based on the simulation results, some possible suggestions to improve the lemang oven arrangement in order for it to produce better quality lemang has been given.
Acknowledgments
The authors would like to thank the Ministry of Higher Education Malaysia for funding this project.
References
[1] N. Therdthai, W. Zhou and T. Adamczak. 2-D CFD Modeling and Simulalation of an Industrial Continous Bread Baking Oven. Journal of Food Engineering 60 (2003): 211-217.
[2] D.C. Saxena, P. Haridas Rao, and K. S. M. S Raghava Rao, Analysis of Modes of Heat Transfer in a Tandoor Oven. Journal of Food Engineering 26 (1995) 209-217.
[3] M. Boulet, M. Bernard, M. Dostie and C. Moresoli. CFD Modeling of Heat Transfer and Flow Field in a Bakery Pilot Oven. Journal of Food Engineering 97 (2010) 393-402.
