Heat transfer through metal

[roadrunner]

I have a limited understanding of heat flow through metal, so am hoping that someone here may be able to clear up my confusion.
If I take a 2 foot length of 1/2 inch solid copper rod, stand it up vertically and apply a blowtorch to the centre of the rod the heat travels upwards and chances are I will burn my fingers if I try to pick it up from the top, yet the bottom of the rod will stay cool. I could put the vertical transfer of heat down to air currents, but I'm thinking that if the rod was in a vacuum then the heat would still rise upward, especially when I look at the (partially?) evacuated solar water heater tubes that are now commonplace.
So what happens inside an i.c.e. when there is a high heatload in the piston? What percentage of the heat rises to (or stays in) the piston's top section (to be transfered to the cylinder wall through the rings and into the inlet charge), and how much heat travels down into the bigend? Anyone?

[Greenlight]

In your 2' long rod example there are a couple of things going on.

1) The center of the rod heats up because an energy source (torch) is applied. Duh!!!

2) The top of the rod experiences heat transfer due to conduction (the interaction of the molecules in the rod), the interaction of the "still" air with the rod, and free convection (the air in contact with the rod rising due to density differences between the cool air and hot air). The free convection takes place along the entire portion of the rod that is at a higher temperature than the surrounding air. The rising heated air at the lower portion of the rod adds to the free convection heat going all the way up the rod. So, the top of the rod is much hotter than the bottom.

3) The bottom of the rod (if touching a surface other than the air, let's say a steel plate) will conduct some of the heat to the steel plate through "contact" resistance (in other words it is not welded to the plate and there is likely some small air space between the rod and the plate that restricts "pure" conduction heat transfer to take place. So, some of the heat energy in the lower portion is transferred to the steel plate (the value of contact resistance determines how much) and some to the air (through conduction with the air and through free convection).

Radiation heat transfer (from the torch flame) adds a little to the heat transfer, but probably not much.

If you held the rod horizontal, I would make an educated guess that you could hold the rod in your hand if your hand was placed 15" to 18" away from the heat source (torch).


Heat distribution in a piston:
http://www.jatit.org/volumes/Vol48No2/37Vol48No2.pdf

Heat distribution in the cylinder wall:
http://web.mit.edu/16.unified/www/FALL/ ... de123.html

[roadrunner]

Thanks for the reply/links Greenlight, I was really after a better understanding of heat flow through the conrod of a running engine, the telltale bluish tone on overheated small and big ends always seems to be fairly localised, so am I to assume that crankcase air/oil mist is at work pulling heat from the more central area of the conrod?

[Dan Timberlake]

the popular statement that "heat rises" is mostly about bouyancy effects of gases and liquids in the presence of gravity here on earth.

The heating of the vertical copper bar in your example would be the same upward and downward if the bar was wrapped with insulation, to keep the air heated by the flame from rising and flowing over the upper half, or if another heat source was used, like inductive or radiant.

[Kevin Johnson]

Thanks for the reply/links Greenlight, I was really after a better understanding of heat flow through the conrod of a running engine, the telltale bluish tone on overheated small and big ends always seems to be fairly localised, so am I to assume that crankcase air/oil mist is at work pulling heat from the more central area of the conrod?

Yes, confining the discussion to the little and small ends. The big end can overheat simply by changing the flow field and thereby the residency time of the lubricant.

But do not assume that heat is not generated locally on the beam of the rod. Heat is generated by the flexing of the rod -- it is not a theoretically rigid construct. If there are flaws or stress raisers that area can easily overheat and leave witness marks or color change. You are probably just examining well designed rods for their application. Cue the I and H beam discussion -- stage left.

The idea of residency time holds true for windage control as well. You want a continual flux of renewed liquid to reject heat but you must have an efficient means of then removing that heated "coolant" fleeing the surface by rivulets and droplets before it switches roles and becomes a vehicle for heat generation through impact friction. A ballet.

[roadrunner]

the popular statement that "heat rises" is mostly about bouyancy effects of gases and liquids in the presence of gravity here on earth.

The heating of the vertical copper bar in your example would be the same upward and downward if the bar was wrapped with insulation, to keep the air heated by the flame from rising and flowing over the upper half, or if another heat source was used, like inductive or radiant.

Thanks Dan, that info will help me to build my own evacuated solar heating rods of I ever get around to it!

[roadrunner]

am I to assume that crankcase air/oil mist is at work pulling heat from the more central area of the conrod?

Yes, confining the discussion to the little and small ends. The big end can overheat simply by changing the flow field and thereby the residency time of the lubricant.

So maybe sticking to the engine manufacturers designated oil weight according to ambient temps would be a good idea? (20/50 during summer, 15/40 winter). If I throw some 5w30 synthetic into the close to stock engine is the faster flow of oil likely to pull less heat from the bigend due to less residency time, or will the lower weight oil create less friction/heat and therefore allow the bearing to run cooler? "There are probably too many variables involved for you to give me a straight answer, but have a go anyway

But do not assume that heat is not generated locally on the beam of the rod. Heat is generated by the flexing of the rod -- it is not a theoretically rigid construct. If there are flaws or stress raisers that area can easily overheat and leave witness marks or color change. You are probably just examining well designed rods for their application. Cue the I and H beam discussion -- stage left.

I have just finished reading the 6 pages of I vs. H beam rods, interesting, and application dependent from what I can decipher.

The idea of residency time holds true for windage control as well. You want a continual flux of renewed liquid to reject heat but you must have an efficient means of then removing that heated "coolant" fleeing the surface by rivulets and droplets before it switches roles and becomes a vehicle for heat generation through impact friction. A ballet.

Noted!

[roadrunner]

=Kevin Johnson The idea of residency time holds true for windage control as well. You want a continual flux of renewed liquid to reject heat but you must have an efficient means of then removing that heated "coolant" fleeing the surface by rivulets and droplets before it switches roles and becomes a vehicle for heat generation through impact friction. A ballet.

How does crankcase gas evacuation affect heat removal? Should I create a vacuum in the crankcase?

[Kevin Johnson]

=Kevin Johnson The idea of residency time holds true for windage control as well. You want a continual flux of renewed liquid to reject heat but you must have an efficient means of then removing that heated "coolant" fleeing the surface by rivulets and droplets before it switches roles and becomes a vehicle for heat generation through impact friction. A ballet.

How does crankcase gas evacuation affect heat removal? Should I create a vacuum in the crankcase?

If you create a vacuum the distribution pattern of the droplets will change because of the lack of a gas carrier (windage). A faux windage can be created by droplet impacts. Short answer is that much more careful design of the components and oiling system will be required because the changed distribution pattern which will affect both the lubricant and coolant functions demanded of the oil.

[MadBill]

Thanks for the reply/links Greenlight, I was really after a better understanding of heat flow through the conrod of a running engine, the telltale bluish tone on overheated small and big ends always seems to be fairly localised, so am I to assume that crankcase air/oil mist is at work pulling heat from the more central area of the conrod?

I would say 99.9% of blued small ends are a result of heating them to install pressed pins and 100% of blued big ends are due to bearing/lubrication issues. A steel surface turns blue at ~ 500° F., far above the capability of the oil or the operating range of anything else in the bottom end of an engine not engaging in self-destruction.

[Kevin Johnson]

Some connecting rods do not use pressed small end pins and have heat discoloration. Roller bearings are apparently more prone to this type of issue.

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[MadBill]

Glad I said 99.9%, instead of 100% Kevin!

BTW, those pix certainly show heat effects, but in most it looks to be baked on oil constituents. Steel officially turns blue in air at 575°F. but yellow at 'only' ~ 450° F. Also one set appears to be aluminum which doesn't change color with heat.

[Kevin Johnson]

Glad I said 99.9%, instead of 100% Kevin!

BTW, those pix certainly show heat effects, but in most it looks to be baked on oil constituents. Steel officially turns blue in air at 575°F. but yellow at 'only' ~ 450° F. Also one set appears to be aluminum which doesn't change color with heat.

Spent several hours reading papers. I think that if you want to "see" discoloration due to heat damage to aluminum you need non-human peepers.

Here is a recent paper that suggests that location specific changes in the aluminum properties of a part from location specific thermal histories will be "visible" to ultrasonic examination.

http://www.hindawi.com/journals/amse/2013/407846/

Research Article
Evaluation of Ultrasonic Nonlinear Characteristics in
Heat-Treated Aluminum Alloy (Al-Mg-Si-Cu)

JongBeom Kim (1) and Kyung-Young Jhang (2)
1 Graduate School of Mechanical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
2 School of Mechanical Engineering, Hanyang University, Seoul 133-791, Republic of Korea

Correspondence should be addressed to Kyung-Young Jhang; kyjhang@hanyang.ac.kr
Received 19 July 2013; Revised 7 October 2013; Accepted 21 October 2013
Academic Editor: Young Soo Choi

Copyright © 2013 J. Kim and K.-Y. Jhang. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.

Abstract
The nonlinear ultrasonic technique has been known to be more sensitive to minute variation of elastic properties in material than
the conventional linear ultrasonic method. In this study, the ultrasonic nonlinear characteristics in the heat-treated aluminum
alloy (Al-Mg-Si-Cu) have been evaluated. For this, the specimens were heat treated for various heating period up to 50 hours
at three different heating temperatures: 250∘C, 300∘C, and 350∘C. The ultrasonic nonlinear characteristics of each specimen were
evaluated by measuring the ultrasonic nonlinear parameter [beta] from the amplitudes of fundamental and second harmonic frequency
components in the transmitted ultrasonic wave. After the ultrasonic test, tensile strengths and elongations were obtained by the
tensile test to compare with the parameter [beta]. The heating time showing a peak in the parameter [beta] was identical to that showing
critical change in the tensile strength and elongation, and such peak appeared at the earlier heating time in the higher heating
temperature. These results suggest that the ultrasonic nonlinear parameter [beta] can be used for monitoring the variations in elastic
properties of aluminum alloys according to the heat treatment.

I would write more but do not have the time to compose a paper on the topic.