CATEGORIES FOR ARTICLE LINKS BELOW

As we have discussed, cooling air becomes warmer as it travels through the radiator core. Each successive row of tubes becomes cooled by warmer and warmer cooling air until at some point little or no heat transfer takes place. As was discussed regarding fin count, in every installation there is an optimum combination of fan performance and core restriction that will produce maximum heat transfer performance. Increasing the core restriction from this point by increasing the number of rows of tubes will reduce the heat transfer performance of the radiator. However, if there is a high rate of cooling airflow through the core, adding a row of tubes will probably provide some improvement. In high performance applications with louvered fins, three rows or a maximum of four rows will probably provide best performance. Increasing beyond four rows in a louvered core will provide little or no improvement and may even result in reduced performance.

Adding another row of tubes has other effects. It provides another path for the coolant, resulting in lower coolant flow velocities through the tubes. Optimum coolant flow velocity through the radiator tubes is about 6 to 8 feet per second. If the flow rate becomes low enough, laminar flow occurs, creating a boundary layer of coolant along the walls of the tubes. This boundary layer, or very slowly moving layer of coolant, acts as an insulator and retards heat transfer. Going to a smaller tube size when adding a row of tubes is one way to keep the coolant flow rates up in the tubes to help prevent the formation of a boundary layer. Another way is to use dimpled tubes, which are commonly used in low flow applications.

Contrary to popular opinion, dimpled tubes do not slow the coolant down in order to make it stay in the tubes longer. The dimples increase the length of the coolant flow path by making the coolant twist and turn as it passes through the tube. This actually speeds up the coolant flow along the tube wall, increasing its “scrubbing action,” preventing the formation of a boundary layer, and improving heat transfer. On the other hand, using dimpled tubes when they are not needed can hurt heat transfer performance by increasing tube restriction, which reduces coolant flow and can cause cavitation at the coolant pump.

-IMPROVEMENT RULE- #7 Adding a row of tubes may help, but it may hurt by increasing cooling air restriction and reducing the coolant flow rate in the tubes. If the cooling airflow has been increased over the original installation, adding a row or two will probably help in this situation. Increasing the number of rows beyond 4 in a louvered fin core will almost always hurt.

-IMPROVEMENT RULE- #8 Adding two rows of tubes without increasing the coolant flow rate (Bigger pump or turning the old pump faster) will probably reduce performance because of low coolant flow rate in the tubes. Reducing the tube size or going to dimple tubes may help. Increasing the coolant flow rate will surely help.

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