The effect of stirrer type on the heat transfer of the cooking system

As was discussed, a high heat transfer rate is required for this cooking system to decrease the temperature difference between the oil and the surface temperature and to approach isothermal conditions at the pan surface. The thickness of the boundary layer

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Te mperature ( ˚ C) Time (min) Oil Edge of pan Center of pan Pastry

Chapter 3: Model food and cooking system development 51 of unmoving fluid (x0) can be decreased by turbulent mixing of the fluid (Equations 3.1

and 3.2). Different stirrer paddle designs for creating turbulent movement in the oil were explored and the types and the speed of the stirrers were investigated so that the most appropriate one could be selected. In a mixing system there are two types of mixing that the stirrer normally produces. These are axial and radial flow patterns, where the turbulent movement needs both patterns. Axial and radial flow patterns are shown in Figure 3.19.

(a) (b)

Figure 3.19 The flow patterns in a mixing system: (a) axial flow and (b) radial flow

The flow pattern of the fluid system plays an important role on the energy transfer and it depends on the type of stirrer applied to the system. This study employed five different types of stirrer to create movement in the cooking system. The five different stirrer types are shown in Figure 3.20.

Each type of stirrer was applied to the cooking pan system at different speeds which were 300, 366, 1000, 1550 and 1180 rpm for stirrer type 1, 2, 3, 4 and 5 respectively. The different speeds were the maximum limit for each stirrer type. The experimental study was carried out by using each stirrer in the cooking system, when the system was turned on and the oil was heated up to the set point temperature of 140°C. After the temperature of the pan was constant at 140°C for 8 minutes, the pastry sample was placed on the pan and run for 35 minutes. During the experiment, the temperature of the oil and the surface of the pan were measured.

52 Chapter 3: Model food and cooking system development

(a) stirrer type 1 (Oblique impeller blade stirrer)

(b) stirrer type 2(Anchor stirrer)

(c) stirrer type 3 (Blade disc turbine stirrer)

(d) stirrer type 4 (Blade agitator impeller stirrer)

(e) stirrer type 5

Figure 3.20 The stirrer types investigated to reduce thermal resistance between oil and pan surface

12 mm 65 mm

Chapter 3: Model food and cooking system development 53 All observations for the system temperature and the pattern of oil fluid movement for each stirrer type were recorded. The temperature profiles of the cooking pan systems for the five different types of stirrer are shown in Figure 3.21.

The stirrer type 1 was applied with a maximum speed of 300 rpm. A vortex formed at the higher speeds (>500 rpm) which made the oil spread out from the oil bath. Radial mixing took place in the oil bath to make the oil temperature uniform. The radial flow pattern moves away from the impeller, towards the sides of the vessel. The flow impacts the side and moves in either an upward or downward direction to fill the top and bottom of the impeller. This paddle type also made a big wave with turbulent oil. The thickness of the boundary layer of unmoving fluid (x0) was still large. There was a temperature

difference of 6°C at steady state.

A maximum speed of 366 rpm was applied for stirrer type 2. This gave extreme turbulence in the oil bath, even when the revolution speed was low (300 rpm). The oil splashed out from the oil bath as a big wave was produced. A radial mixing pattern occurred but no axial mixing. Therefore, the temperature between oil and pan’s surface was still high (5˚C).

The stirrer type 3 is known as the Rushton turbine design and it was used at 1000 rpm. This paddle created radial mixing, a simple swirling motion, producing little agitation and a vortex so the oil moved at a high velocity across the surface of the pan and a uniform temperature occurred. The movement of oil was turbulent because of these two mixing types and the high speed of the stirrer (1000 rpm). Nevertheless, the heat transfer coefficient was not high enough as there was still a high temperature difference of 3°C between the oil and pan surface. This was attributed to the small head area of the stirrer, resulting in non ideal total mixing.

The stirrer type 4 generated only axial mixing, leading to a uniform oil temperature, but the turbulent movement was not extreme enough to increase the heat transfer coefficient, even though the rotation speed was very high (1550 rpm). This may be because the contact area across the paddle was small. The difference temperature was found as 3.5°C.

54 Chapter 3: Model food and cooking system development

The stirrer type 5 was applied at 1180 rpm. Both radial and axial mixing occurred. The axial mixing made the oil temperature uniform and the radial mixing of the oil induced a high turbulent velocity across the surface of the pan. In addition, the very high speed, and extreme turbulence created surface renewal in this system. Therefore, the heat transfer coefficient was very high which caused very fast heat transfer from the oil to the stainless steel pan sheet. The temperature difference was reduced to 2°C.

After putting the pastry samples on the pan, all samples showed a temperature drop. Paddle 5 demonstrated the fastest response of temperature compared to the other types of stirrers. It gave the smallest temperature gap between the sample surfaces and the oil and the temperature recovered back to the original temperature more quickly.

Figure 3.21 Temperature profile of the oil and the surface of the pan using different types of stirrer

The findings for the different stirrer types and stirring speed investigation are summarised in Table 3.2. It can be concluded that stirrer types 3 and 5 provided good results with smaller temperature differences. However, stirrer type 5 had the best performance because this paddle created very fast movement (turbulence) of oil across the surface of pan resulting in a small temperature difference between the oil and the

120 122 124 126 128 130 132 134 136 138 140 142 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Te mper ature ( ˚ C) Time (min) Oil Stirrer 1 Stirrer 2 Stirrer 3 Stirrer 4 Stirrer 5

Chapter 3: Model food and cooking system development 55 pan (θ) of 2˚C. This result was the best approximation to an isothermal process. Stirrer type 5 was therefore selected as the stirrer for use in achieving an isothermal pastry temperature.

Table 3.2 Summary of the study on the effect of the stirrer type

Stirrer types

Maximum revolution of stirrer (rpm)

Mixing type Observations

1 300 Radial Temperature difference was so high (6˚C) 2 366 Radial Temperature difference was high (4˚C) 3 1000 Radial Temperature difference was low (3˚C) 4 1550 Axial Temperature difference was high (3.5˚C) 5 1180 Axial and radial Temperature difference was low (2˚C)