Airflow through a radiator with a restriction

1. Nov 19, 2009

blackman

Hello, I have a question about airflow through a car radiator which has a plate on the discharge side with holes cut for fans. There are fans mounted on there but they aren't being used. And the car is moving.

Would the airflow increase or decrease with the plate on there?

We are having a discussion about this and I am fairly sure that it would decrease as it is a restriction on the airflow.

Others have been saying it would increase as there is a slight vacuum created on the discharge side creating more flow.

None of us have any real physics teaching, just your average high school stuff. Sorry if it is a silly question.

Edit: Also I haven't read through the forum rules here, I hope I haven't broken any, please let me know if I have. Thanks again.

2. Nov 19, 2009

Ranger Mike

first of all..WELCOME..you took a major step up the ladder by signing on to this site and I see no violations of any rules....this forum, on my opinion is for those who want to learn about how things work.

I am no professor or doctor but been racing since 1964. Radiators are sized to the maximum engine cooling requirements> The reason electric fans are hung on the things are to dissipate heat in traffic while idling along. Many times they are not in use at high way speeds as the air flow will provide enough flow to cool the cores. the mounting plates will restrict the flow but not enough to impact the cooling, if the fans are engaged when needed. There will be a slight vacuum created in any radiator installation. This is because a concept called Bernoulli's principle. You can google it but basically you have high speed low pressure air entering the radiator cavity. the pressure must equal the same as the low pressure high speed air under the car. So you have a slight vacuum.

For any heat exchanger ( radiator) the rate of heat dissipation varies directly with the mean ( average) temperature difference between the cooling surface and the air stream. ( technically it is the .6 power of the airstream velocity and the .8 power of the air volume through the core). Both thermal efficiency and internal drag are reduced by slowing down the air velocity in the core. This means we need high energy (laminar and high velocity) air coming into the cavity and that we want to slow the air down before it enters the core. In order to provide an extractor effect and to ensure that the exiting air is traveling at or near free stream velocity when it rejoins the freestream, we also want to accelerate the air after it has passed through the cooler and before it rejoins the freestream. To achieve all this we need a properly deigned air DUCT. A lot of wind tunnel time and expense is used to achieve this with minimum drag and to achieved maximum cooling.