Counter current radiator, anyone considered it?

[Tesla]

Been thinking about this for a while, the counter current aka counter flow radiator is the most efficient heat exchanger layout.
The cooling flow enters where the cooled flow exits and the cooling flow exits where the cooled flow enters, this allows for maximum transfer of heat and a much smaller radiator can be used.
Cars have a crossflow radiator, the two flows are perpendicular, the air passing through the top of radiator works well and removes lots of heat, but the air passing through the bottom is mostly wasted because temperature gradient is very low.

With airflow already restricted with a grill block, I wonder if it would be more effective to actually block off the bottom half of the radiator and force twice as much air through the top half which has a much higher temp gradient.

The paper below talks about a number of aspects of radiator design and also has some discussion on thermal managment in motor vehicles in it's introduction.

http://www.energy.lth.se/fileadmin/e...Lin_110928.pdf

[JRMichler]

Reference #25 in that paper, "Compact Heat Exchangers", by Kays and London, is a very good book for anybody who wants to know why automobile radiators are designed the way they are.

Counterflow works best when the flow rates are low, and temperature changes are high. When the flow rates are high, and temperature changes are low, then a cross flow radiator works almost as well as a counterflow. In an automotive application, a cross flow radiator is cheaper than a slightly smaller counterflow radiator.

[jeff88]

With the cooler flow at the bottom of the radiator, does it make sense to have the grill block lower and have venting inside of the bumper force air upwards towards the higher temperaturs coolant?

I've read most people prefer to block the upper portion of the grill more than lower, but I would think that would hurt cooling and if you do the lower, you can actually block more.

[Tesla]

Originally Posted by JRMichler

Reference #25 in that paper, "Compact Heat Exchangers", by Kays and London, is a very good book for anybody who wants to know why automobile radiators are designed the way they are.

Counterflow works best when the flow rates are low, and temperature changes are high. When the flow rates are high, and temperature changes are low, then a cross flow radiator works almost as well as a counterflow. In an automotive application, a cross flow radiator is cheaper than a slightly smaller counterflow radiator.

I imagine that is when excess air is considered free with cars so a high flow is no concern, but when the aero cost is factored in and airflow is reduced, then maybe we need to also consider how it is directed through the radiator to gain the highest advantage from the lower flow.

Originally Posted by jeff88

With the cooler flow at the bottom of the radiator, does it make sense to have the grill block lower and have venting inside of the bumper force air upwards towards the higher temperaturs coolant?

I've read most people prefer to block the upper portion of the grill more than lower, but I would think that would hurt cooling and if you do the lower, you can actually block more.

The aerodynamics are maximised by blocking upper grill and drawing air in from lower grill, this reduces the amount of air going under the vehicle, the slightly longer air path length is not a problem as this is alowing the air to reduce velocity and increase pressure at the radiator face.

Directing more air through the hotter/upper portion of radiator rather surely must lead to a more efficient outcome than just pushing it evenly through the entire core.

[RedDevil]

I dislike the idea of blocking any part of the radiator (not the grille) for the simple reason that the thermostat regulates the amount of hot water that goes in.
If it lets so much in that the heat reaches the lower regions (if maybe you overblocked the grille?) then it needs the cooling it can get in that area. Blocking it reduces the safety margin.

If you do not need the lower part of the radiator at all, then the best solution would be to get a smaller and lighter radiator.

[Tesla]

Well grill blocking is already at that safety margin, the idea of partial blocking of lower/cooler portion of radiator is simply to maximise heat extraction with the lower airflow, so in actuality would be increasing safety margin.

[JRMichler]

Here's a comparison of cross flow and counterflow heat exchangers. The vertical axis is effectiveness, where 100% effectiveness is 100% heat transfer on the lowest flow side. The horizontal axis is a heat exchanger parameter that includes flow rate, heat exchanger size, and heat transfer coefficients. The multiple lines cover a range of flow conditions. Cmin/Cmax = 0 is roughly the case of full air flow and a light trickle of water flow. With a reasonable size radiator, the water will be cooled down to the outside air temperature (100% effectiveness). Cmin/Cmax = 1.00 is close to the case of maximum cooling - pulling a trailer uphill on the highway on a hot day. Even with the largest radiator, the water will not be cooled down to outside air temperature (less than 100% effectiveness).

The first graph is for cross flow radiators, such as automobile radiators.



This one is for counterflow heat exchangers.



While the counterflow heat exchanger is noticeably more effective than the cross flow, a slightly larger cross flow radiator takes out the same amount of heat. The larger cross flow radiator is cheaper than the smaller, more complex, counterflow radiator.