Cooling solutions for an electronic equipment box operating on UAV systems under transient conditions

Abstract

The thermal behaviour of an electronic equipment box under transient conditions is investigated. The box consists of a cabinet designed to contain electronic devices (such as infrared and visible cameras, hyperspectral sensors, etc.) carried on UAV systems and thermal control has to be assured only for the short duration (15–20 min) of each flight. Experiments were performed by considering a single component as electronic unit and different cooling systems, including forced convection of external or internal air and use of phase change/storage units (cold gel packs) inside the box. The best cooling solution was obtained by placing cold gel packs inside the box in combination with the internal ventilation, without any air exchange with external environment. A simple mathematical model was devised to simulate the thermal response of the component during the cooling transient and calculated results were successfully compared with experimental results. As a result, it is expected that the developed model could be used as a reliable tool for transient thermal management of an array of components inside the box, as encountered in UAV remote sensing applications.

Introduction

Unmanned Aerial Vehicles (UAVs) or simply drones are nowadays widely used for remote sensing in different technological fields, such as environmental monitoring, precision agriculture, humanitarian localization and rescue, photogrammetry, prevention of events (floods, fires, landslides), weather tracking, inspection of power lines and pipelines, surveillance, and so on [[1], [2], [3], [4], [5], [6], [7], [8]]. Visible and thermal cameras, multispectral and hyperspectral sensors, and laser scanners are typical instruments installed onboard of such platforms which offer the opportunity of acquiring spatial and temporal high resolution data. From the point of view of sensors onboard UAVs, payload requirements are an issue to be considered to ensure the success of remote sensing missions; the smaller platform will be more limited for payload, directly affecting the types of sensors that can be transported and thus affecting the attributes of the remote sensing application [2].

Not long ago, remote sensing systems were based on a single sensor, such as a large format camera on UAV or airborne platforms. With advancement in sensing and computer technologies, sensors have become more affordable, and modern remote sensing systems use multiple sensors. Nowadays, numerous sensors are simultaneously present onboard UAVs; some of them are used to capture data with the exclusive aim of controlling the platform during navigation, others are part of the remote sensing system and dedicated to a specific application. Sensors used for remote sensing have to be adapted to the platform and should not be a serious impediment for maneuverability; their synergic use may suggest their lodging inside a box or cabinet, mounted tightly underneath the drone, as shown in Fig. 1. The installation of sensors inside a box makes it easy to add and remove them and can provide protection from collisions and crashes (if made of carbon-fiber walls, for instance) and against the atmospheric agents (rain, wind, dust, solar radiation, etc.).

The sensors placed inside the box generate heat that must be dissipated to prevent failures and improve long term reliability. At the same time, any additional heat input (for instance solar radiation heating) has to be suppressed or minimized. The cooling system must be simple, cheap and without significant mass and volume increases since size and weight are essential issues for commercial, light UAVs. These vehicles typically fly at low altitudes with a short flight duration because of their limited battery capabilities [9]. This means that the thermal control of instrumentation inside the box has to be guaranteed within a little time interval (say, 15–20 min) corresponding to the typical duration of a single flight. For this reason, sophisticated and/or relatively heavy and expensive cooling systems such as heat sinks, heat pipes or jet impingement, typically designed to ensure a long-term thermal control of electronic equipment, are not strictly necessary if the electronic devices are working and dissipating heat only for few tens of minutes.

Forced convection cooling of external air is one of the most common techniques used in electronics; due to the greater and greater complexity of electronic systems, it is essential for their thermal management a careful design of the cooling system based on air-moving devices [[10], [11], [12], [13], [14], [15], [16]]. The search of an optimal configuration by determining the location and orientation of fans inside a computer chassis was numerically faced by Wang et al. [11]; the authors concluded that, despite an overall optimal chassis configuration in terms of heat dissipation does exist if only one specific element is under consideration, general guidelines for enhancement of heat dissipation include the development of a rather long main air stream between the fan and the exit in order to carry more energy away and the installation, in a rather cool environment, of fans blowing air into the chassis than sucking air out of the chassis. A cooling configuration consisting of multiple miniature axial fans impinging air on finned surfaces has been experimentally and numerically investigated by Stafford and Fortune [12], who demonstrated that fans positioned adjacently in an array can influence heat transfer performance both positively and negatively by up to 35% compared to an equivalent single fan-heat sink unit operating standalone. Eliand et al. [15] experimentally tested arrays of fans having different size within the chassis of web servers, finding that 80 mm and 120 mm fans consolidated to the back of a rack performed better than smaller, 60 mm baseline fans.

Another cooling technique is to take advantage of phase change or heat storage materials that could potentially damper temperature oscillations and rises of electronics during intermittent or transient conditions [[17], [18], [19], [20]] or contribute to maintain, for a given amount of time, some items or devices at a relatively low temperature for pharmaceutical and food conservation appliances [21,22]. A phase change material can store the energy during temperature rises by absorbing, during its melting, the heat from the surroundings; after that, when the temperature decreases below its phase change temperature, it then releases the heat energy and goes back to its initial solid phase. In particular, gel packs are phase change materials composed primarily of water, sometimes with additives to decrease the phase change temperature. Reusable gel packs may represent a practical and cheap solution to control the temperature of an electronic system when the thermal load is intermittent; the only disadvantage is that, after their use, gel packs have to be kept in a cool environment to return to a solid state.

This study is focused onto the search of effective cooling modes of sensors located inside a box under working conditions, targeted for the specific UAV application. Since electronic devices used for UAV remote sensing may largely differ in mass, volume, dissipated power and geometrical distribution inside the box, the identification of the best cooling solution cannot be pursued and generalized for any assembly of sensor devices. For this reason only one specific and simple configuration (a prismatic three-dimensional heated plate inside an enclosure) has been considered and experimentally tested, as described in Section 2, in order to investigate its thermal behaviour in dynamic conditions according to different cooling strategies. To complement the experimental activity, which constitutes the main body of the work, a lumped-parameter model has been developed and illustrated in Section 3. Experimental and calculated thermal results for the basic configuration are outlined in Section 4, where calculation examples are also provided for a real box equipped with visible, thermal and hyperspectral cameras for remote sensing applications. Finally, Section 5 summarizes main concluding remarks.