23-05-2022, 10:50 AM
(This post was last modified: 26-05-2022, 11:11 AM by Colin Morgan.)
Looked a bit further into thermosyphon cooling to think about what might be going on inside. (If anyone spots errors or numerical mistakes, I will be happy to make corrections.)
The water circuit comprises the block + head, top hose, radiator header tank, radiator tubing and bottom tank/bottom hose back up to the block.
Assuming the cooling system contains just water (no antifreeze) and the engine is putting out a useful 7 BHP – (the car is doing a steady 35mph or so?) then, if the engine is 40% efficient, the power output of 7 BHP (= 5.1KW), gives the radiator 7.6KW of waste heat to dissipate at equilibrium, ignoring other heat losses. (Correction - this amount of heat being lost by the radiator would be from about 1/3 of the total output of the engine - with in practice about 1/3 of the energy from combustion being useful work, 1/3 being taken away by the exhaust gases, and 1/3 lost through the radiator. So the 7.6KW of heat through the radiator would occur when the engine was putting out 7.6KW of useful work, so about 10 BHP - which would correspond to driving on the level at a speed of about 40 mph.)
If the temperature is measured at the top of the radiator and found to be 80 C, and at the bottom 40 C, then the amount of water in circulation can be calculated. The water flow rate is the KW of heat removed divided by the heat capacity of water x temperature difference top to bottom in the radiator. So, this is 7.6KW divided by (4200J/Kg C x 40 C) which is 0.045 Kg per second (or litres per second), so about 45 g/second or cc/second (or 1.8 fl oz, or about one egg’s volume a second). This isn’t much and shows how good a coolant water is, with its very high heat capacity?
Given a total cooling water capacity of about 11.5 pints, which is 5.4 litres, this flow rate means that the water takes an average of 119 seconds (5.4 litres/0.045 litres/sec), or almost 2 minutes, to go around the whole system.
The top hose is about 40mm diameter. At this flow rate the water is travelling on average at 0.04m/s or about 40mm or 1.5” per second. The radiator has about 45 crimped tubes about 50mm x 2mm in section, which gives an average speed of flow of about 10mm/s down though the radiator.
From the estimated water speeds, time taken by the water to past through various parts of the circuit can be calculated: Top hose about 9 seconds, radiator top tank (if water is 3/4” deep) 18 seconds, rest of radiator about 45 seconds, bottom hose 9 seconds, inside block and head (assuming flow area is 40% of 11” x 5” and it is 5” high) 40 seconds. (This adds up to 121 seconds – which is similar to the estimated time from the heat balance of 119 seconds.)
The water circuit comprises the block + head, top hose, radiator header tank, radiator tubing and bottom tank/bottom hose back up to the block.
Assuming the cooling system contains just water (no antifreeze) and the engine is putting out a useful 7 BHP – (the car is doing a steady 35mph or so?) then, if the engine is 40% efficient, the power output of 7 BHP (= 5.1KW), gives the radiator 7.6KW of waste heat to dissipate at equilibrium, ignoring other heat losses. (Correction - this amount of heat being lost by the radiator would be from about 1/3 of the total output of the engine - with in practice about 1/3 of the energy from combustion being useful work, 1/3 being taken away by the exhaust gases, and 1/3 lost through the radiator. So the 7.6KW of heat through the radiator would occur when the engine was putting out 7.6KW of useful work, so about 10 BHP - which would correspond to driving on the level at a speed of about 40 mph.)
If the temperature is measured at the top of the radiator and found to be 80 C, and at the bottom 40 C, then the amount of water in circulation can be calculated. The water flow rate is the KW of heat removed divided by the heat capacity of water x temperature difference top to bottom in the radiator. So, this is 7.6KW divided by (4200J/Kg C x 40 C) which is 0.045 Kg per second (or litres per second), so about 45 g/second or cc/second (or 1.8 fl oz, or about one egg’s volume a second). This isn’t much and shows how good a coolant water is, with its very high heat capacity?
Given a total cooling water capacity of about 11.5 pints, which is 5.4 litres, this flow rate means that the water takes an average of 119 seconds (5.4 litres/0.045 litres/sec), or almost 2 minutes, to go around the whole system.
The top hose is about 40mm diameter. At this flow rate the water is travelling on average at 0.04m/s or about 40mm or 1.5” per second. The radiator has about 45 crimped tubes about 50mm x 2mm in section, which gives an average speed of flow of about 10mm/s down though the radiator.
From the estimated water speeds, time taken by the water to past through various parts of the circuit can be calculated: Top hose about 9 seconds, radiator top tank (if water is 3/4” deep) 18 seconds, rest of radiator about 45 seconds, bottom hose 9 seconds, inside block and head (assuming flow area is 40% of 11” x 5” and it is 5” high) 40 seconds. (This adds up to 121 seconds – which is similar to the estimated time from the heat balance of 119 seconds.)