13-05-2022, 10:04 PM
(This post was last modified: 13-05-2022, 10:05 PM by Colin Morgan.)
Thinking about how such as system works… namely, by convection - the reduced density of the hot water making it buoyant so it rises. The driving force for the circulation is the average difference in density between the water on the engine side of the circuit with that on the radiator side. As there is no obstruction in the circuit, even a small driving force from a small amount of heat in the block would start the water circulating very slowly, given a bit of time whilst it got established. As soon as the water moves, resistance to this flow starts to build around the system and this resistance counteracts the buoyancy force such that the flow settles at the level where these are equal.
[When the engine is started, water in the block starts to warm, its density drops and its buoyancy makes the hot water rise as high as it can go in the system, which is into the radiator header tank, where it mixes with the water already there. As the total volume of the water in the system doesn't change much as it warms up (expansion even over a temperature range of 60 degrees C is only about 3%), this upward water movement will start to slowly drag cold water up from below the block, and this must then result in the water around the whole circuit starting to move, though only very gradually at first. As more heat goes into the water inside the block and head, the amount of hot water in the top of the radiator increases. Some of this warmer water starts to be pulled down into the radiator where it can start to lose heat. As it travels down the radiator it continues to cool, its density increasing again. Colder water from the bottom of the radiator continues to be dragged round by the movement of the water from the block and head to radiator by the buoyancy force. From the photos, it seems that it takes a bit of time for this circulation to become properly established and there is a short period where an excess of hot water gathers at the top of the radiator before the increased flow rate allows the radiator to perform more effectively.
The flow keeps increasing until an equilibrium is reached where the heat being lost through the radiator equals the (waste) heat produced by the engine (ignoring other direct heat losses), at which point the temperatures stabilise. At equilibrium, the buoyancy force that drives the water around the circuit then equals the resistance to the flow of water through the block, head, hoses and radiator.]
[When the engine is started, water in the block starts to warm, its density drops and its buoyancy makes the hot water rise as high as it can go in the system, which is into the radiator header tank, where it mixes with the water already there. As the total volume of the water in the system doesn't change much as it warms up (expansion even over a temperature range of 60 degrees C is only about 3%), this upward water movement will start to slowly drag cold water up from below the block, and this must then result in the water around the whole circuit starting to move, though only very gradually at first. As more heat goes into the water inside the block and head, the amount of hot water in the top of the radiator increases. Some of this warmer water starts to be pulled down into the radiator where it can start to lose heat. As it travels down the radiator it continues to cool, its density increasing again. Colder water from the bottom of the radiator continues to be dragged round by the movement of the water from the block and head to radiator by the buoyancy force. From the photos, it seems that it takes a bit of time for this circulation to become properly established and there is a short period where an excess of hot water gathers at the top of the radiator before the increased flow rate allows the radiator to perform more effectively.
The flow keeps increasing until an equilibrium is reached where the heat being lost through the radiator equals the (waste) heat produced by the engine (ignoring other direct heat losses), at which point the temperatures stabilise. At equilibrium, the buoyancy force that drives the water around the circuit then equals the resistance to the flow of water through the block, head, hoses and radiator.]