The air flow characteristics within the hive changed significantly when a bottom cavity was added to the hive. The parameter δ defines the bottom cavity depth and the case of δ = 0 mm represents the standard flat bottom board found in the traditional hive geometry. For this case, the air entering the hive flows to the back and is forced up. A fraction of the flow stream penetrates the upper hive compartment while the remainder, once meeting the warm hive air, is redirected and forced back out through the inlet. Adding a bottom cavity to the hive allows the cooler air entering the hive to settle quickly into the bottom. The flow then travels along to the back of the hive where it is forced up to meet the warm hive air and most of the flow is pushed back out the front of the hive. Figure 4.1 shows the evolution of the flow field with increasing bottom cavity depth. The most significant feature of the flow field is the vortex which develops at the front of the hive, which becomes clearly defined once δ = 70 mm. In all cases, the driving force for air flow is the temperature gradient (∆T) created between the warm brood (maintained at 33 → 35 ◦C) and the outside ambient temperature Tamb. For cases where the temperature gradient was relatively large (Tamb < 25 ◦C) increasing the depth of the bottom cavity resulted in a significant increase of the ventilation flow-rate. As illustrated in Figure 4.2, the increase in ventilation flow rate follows an exponential trend that reaches a maximum once the bottom cavity is extended to a depth of δ = 70 mm. The largest increase in ventilation flow-rate was observed for the cases with the lowest ambient air temperature (Tamb = 17.5 ◦C), which correspond to cases having the largest temperature gradient and the largest driving force. As the bottom cavity depth was increased from δ = 0 → 90 mm the ventilation flow rate across the hive inlet increased from 3.5L/min → 6.0L/min. At the opposite extreme, for cases where the temperature gradient was small (Tamb → 27.5 ◦C), the trend of increased ventilation flow-rate with increased cavity-depth was observed to reach a maximum at δ = 50 mm and slowly drop off as the cavity depth was extended further. Flow at the inlet of the hive featured an influent draft at the bottom of the inlet gap and an effluent draft at the top. The strength of the flow into and out of the hive increases as the parameter δ is increased. Figure 4.3 illustrates changes in the flow behaviour by considering velocity magnitude in the Y-direction, the direction normal to the inlet plane, plotted along the span of the inlet in the vertical direction (z-direction) when Tamb = 25 ◦C. The air exchange is weakest when δ = 0 mm and the separation of inflow and outflow occurs at approximately z = 20 mm, where z = 30 mm represents the top of the inlet. The air exchange across the inlet is strongest when δ = 70 mm and the separation of inflow and outflow occurs at approximately z = 16 mm. As the depth of the bottom cavity increases, flow across the inlet is enhanced and height of the separation of inflow and outflow decreases slightly. The amount of CO2 generated in the hive remained relatively constant for all cases of bottom cavity depth. This is expected since the thermal input for the model are the temperature profiles applied to represent the brood, which is fixed for all cases. Considering Figure 4.4, it is interesting to note the substantial changes to the clearance efficiency of CO2 (η) that occur as the parameter δ is varied. When the driving force for flow (∆T) was large, the influence of bottom cavity depth on the parameter η was less pronounced – a very small increase in clearance efficiency was observed, which began to drop off slightly as δ was increased beyond 50 mm. For cases where ∆T was small, the influence of bottom cavity depth became more apparent.

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