Aerodynamics Fundamentals: Heat Exchangers
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Heat Exchangers
05.44
| 00:00 | Racecars require quite a lot of cooling to keep engines, gearboxes and differentials all functioning at their best, so in this section of the course we're going to dive into how we should deal with cooling functionality when designing the aerodynamics of a vehicle. |
| 00:15 | To clarify, we'll be discussing the aero aspects of cooling ducting, not the sizing and selections of the coolers themselves. |
| 00:23 | It's not unusual to see a radiator, engine oil cooler, intercooler, differential cooler and gearbox cooler at play on a car, and all of these need to be working properly for a car to keep its temperatures under control, especially for categories like endurance racing. |
| 00:39 | When you think of something like a car's radiator, this is commonly referred to as a heat exchanger or cooler. |
| 00:46 | Heat exchangers in cars usually take the form of a series of tubes with wavy fins between them. |
| 00:52 | The hot fluid that needs to be cooled, such as water or oil, is pumped through the tubes and the cooling fluid, which is typically the air flowing over the car, flows between the tubes and in the direction of the fins. |
| 01:07 | The fins are connected to the tubes and this allows heat to conduct to the fins from the tubes. |
| 01:13 | The increased surface area of the fins versus just the plain tubes presents more area to the fluid passing through, and this increased surface area means more heat transfer between the fluid and the metal of the heat exchanger. |
| 01:26 | The hot fluid puts heat into the metal and then the cold fluid takes it out, allowing the exchange of heat between the two fluids. |
| 01:34 | With any form of heat transfer, the rate at which it occurs is linearly related to the temperature difference between the objects that the heat is being transferred between. |
| 01:43 | If the temperature difference is double and the geometry is kept the same, the heat transfer rate will also double. |
| 01:50 | So if we have 50 degree fluid going through a heat exchanger and we pass through 20 degree air, it will be twice as effective as having a 50 degree fluid passing through 35 degree air. |
| 02:01 | There are, however, some nonlinearities involved here in the real world. |
| 02:06 | The key one is that as our air passes through our heat exchanger, it heats up, so towards the back of the heat exchanger, the air will be hotter and less effective at cooling. |
| 02:17 | Similarly, the fluid being cooled will get cooler as it passes through the heat exchanger, and this will make it harder and harder to cool as we get towards the cold side. |
| 02:27 | The first scenario is improved by increasing the flow rate of the air through the heat exchanger. |
| 02:32 | As the air stays cooler for longer through the exchanger, thus the rate of heat transfer is higher by the back of the heat exchanger. |
| 02:40 | So flowing more air mass flow through the heat exchanger usually means a more powerful and effective exchange of heat. |
| 02:46 | We can see, though, that this effect will cause the rate of heat transfer to be nonlinear with respect to mass flow. |
| 02:53 | Above a certain velocity, we won't start to see a huge change in the rate of heat transfer through the heat exchangers, so it's definitely not a case of doubling the speed through the heat exchanger, doubling the cooling rate. |
| 03:04 | One thing that does increase quite significantly as we increase mass flow through the heat exchanger is the amount of loss. |
| 03:11 | All the additional surface area on a heat exchanger gives us extra boundary layer growth all over the radiator which is discarded as loss downstream. |
| 03:20 | This means that we lose a lot of energy from our air that results in more car drag overall, and poorer performance of downstream devices such as rear wings. |
| 03:30 | Another consideration for heat exchangers is the placement of each heat exchanger with respect to one another. |
| 03:36 | It is quite common to have a heat exchanger setup where we need to stack cores one behind another due to packaging constraints. |
| 03:42 | For example, most turbo touring cars will end up with an intercooler stacked in front of their water radiator. |
| 03:49 | The most important thing to note if we have to go for a stacked setup is that we need to prioritize the coldest fluid at the front to ensure the heat rejection is as efficient as possible. |
| 03:59 | Remember, the rate of heat transfer is proportional to the difference in temperatures. |
| 04:04 | We don't want to have a 90°C water radiator behind a 110°C oil radiator as that will cause the hot air off the oil cooler to then be hitting the water radiator, which will then have much less temperature differential, whereas if we did this the other way around, it would work much better. |
| 04:20 | An exemption to this rule is turbo intercoolers, which have a high working fluid temperature, but will be often stacked in front. |
| 04:28 | The reasons for this are 1.) Air, the working fluid for the turbo, has less heat capacity, so it isn't putting as much heat into the heat exchanger and it cools down quickly. |
| 04:38 | 2.) The exit temps of the intercooler are quite low, relatively speaking, because the air is losing temperature so fast. |
| 04:46 | And 3.) Getting cold intake air temperatures is critical for performance, as opposed to radiators that are just required to keep all the systems alive. |
| 04:55 | That covers the basics of heat exchangers, so let's run over the main points again before moving on. |
| 05:00 | Cooling in race cars is critical to maintain optimal performance for components like the engine, gearbox and differential, especially in endurance racing. |
| 05:09 | Heat exchangers such as radiators and oil coolers use air flowing over tubes with fins to transfer heat from hot fluids like water or oil. |
| 05:18 | The efficiency of heat transfer depends on factors like temperature differences and air mass flow, with increasing airflow increasing cooling, but also causing more drag and flow energy loss. |
| 05:29 | In stacked heat exchanger setups, placing the cooler fluid at the front maximizes efficiency except in cases like turbo intercoolers which prioritize cold intake air temperatures. |
