The thermal behaviour of segregated heat sink structures in natural convection

Remotely deployed wireless devices such as tower-top active antenna arrays, remote radio heads, and pico or femto-cells are an increasingly prevalent feature of communications systems. Environmental standards which govern outdoor wireless equipment stipulate stringent conditions: high solar loads (up to 1 kW/m2), ambient temperatures as high as 55ºC and negligible wind speeds (0 m/s). These challenges result in restrictions on power dissipation within a given envelope, due to the limited heat transfer rates achievable with natural convection. There are two main objectives of this thesis; the first is to compare the results from a numerically modelled natural convection heat sink, representative of a remote radio head, with and without a solar shield subject to industrial standards and an environmentally defined worst case condition (a daytime condition in a hot location). The impact of solar shield length and location of the heat sink within the solar shield are investigated. The second main objective of this thesis is to segregate the components within a remote radio head across two heat sinks to improve the heat dissipation of the temperature sensitive components. The primary heat sink, representative of the temperature sensitive components, is experimentally and numerically investigated with and without a shield and without wind or solar loading. A secondary heat sink, representative of high power devices, is added to the shielded configuration in order to create a segregated structure. The secondary heat sink is investigated to determine the impact of the location and the number of fins to maximise the heat transfer on the primary heat sink. The deployment of a solar shield which is longer than the device that it contains was found to increase heat dissipation through a ‘chimney flow’ effect and it was determined that the heat sink should be located at the base of the solar shield. For a segregated structure, a secondary heat sink with a base plate temperature of 65°C above ambient was found to increase the Nusselt number of the primary heat sink by over 50 % in comparison to an unshielded case. The optimization of the number of fins on the secondary heat sink was found to offer an enhancement of 80 % over an unshielded heat sink. The findings of this thesis are of practical relevance for the thermal design of outdoor communications equipment: in particular, it is evident that the use of standard environmental conditions is conservative in comparison with a more comprehensive design process which references representative worst case data.