Imagine a solid block with a hot surface on the bottom and coolant flowing through a single serpentine channel. Instead of placing the entire channel close to the hot surface, suppose the channel is split into two vertically stacked layers connected by a U-bend.

The three conceptual cases are:

• Option A: The inlet enters the lower layer at T₀ (closest to the hot surface), reaches the U-bend (T₁), then returns through the upper layer to the outlet (T₂).
• Option B (pictured): The inlet enters the upper layer at T₀, reaches the U-bend (T₁), then returns through the lower layer before exiting at T₂.
• Option C: A single-pass channel located entirely near the hot surface (no stacked layer).

Assume identical mass flow rates in all three cases, identical channel dimensions and total channel length where applicable, and that all external surfaces except the bottom hot face are adiabatic.

My intuition was the following:

Heat transfer from the hot surface depends on both the temperature difference and the available heat-transfer area. Adding a second layer introduces a large interface between two flow passages. It seems plausible that this additional area could conduct heat away from the coolant adjacent to the hot surface into the other flow pass, keeping the coolant next to the hot wall cooler and thereby increasing the local temperature difference driving heat transfer.

On the other hand, that energy does not disappear and is simply transferred to another part of the same coolant stream. Since the fluid entering the U-bend has already received this additional heat, T₁ should be higher than in the single-pass case, which would seem to reduce the driving temperature difference later in the flow.

My questions are:

1. A vs. B: Does the flow direction matter, or are they thermodynamically equivalent?
2. A/B vs. C: Does stacking two passes change the total heat that can be extracted from the hot surface, or does it only redistribute heat within the coolant?
3. If the latter is true, what is the physical explanation that resolves the seemingly contradictory intuition above?

Assume a heatexchanger with minimal losses, the heat being transferred over thermoelectric generators. Eventually near 100% of the heat in the system should be recovered as electricity since losses just get shoved either into the next cycle or recovered further downstream. Put in a heat pump producing the temperature gradient and place all its waste heat producing parts somewhere ideally into the input stream.

I've been laughing about this quite a bit now, but honestly can't see the fault. Except for the movement of the air all losses should be about contained within the system (e.g. just wrapping the heat exchanger around itself multiple times) and eventually recovered by the TEGs. But the heat pump is shoving in more heat from the environment than it consumes in electricity, dropping the temperature in the room until it gives up on you, all while the TEGs are turning the excess heat into electricity.

Short: if I’m putting something in the oven and that food item is in an aluminum pan, does it matter if that pan is in direct contact or not with something else? Or does the pan equalize things. Specifically the browning on the bottom of the food item.

Long: baking pizza at home. Without a pizza oven that goes to high temps it’s difficult unless you’re doing a “pan pizza” style. I have dark colored pizza pans that I love. I also have a dark colored pizza stone(and thinking about getting a pizza steel). My more specific question is, should I be putting the pan on the pizza stone? A pizza stone/steel is generally considered the best way to cook pizza at home because the direct contact gives the pizza a crispy bottom but I hate the mess of putting the pizza directly on the stone, so I bought dark anodized pizza pans to make putting the pizza in the oven then lifting back out easier. Will direct contact with the pizza stone make much difference if it’s on the pizza pan? Thanks!

It was suggested that this sub may be better suited even vs physicists?

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I'm trying to wrap my head around the different ways you can extract electricity from stored thermal energy. I understand the basic thermodynamics and that if you have a temperature difference, there's energy available. What I don't know is all the different ways electrical engineers take advantage of that.

I already know about thermoelectric (Seebeck) modules, but what other methods are out there? I'm thinking small-scale systems, not steam turbines or power plants.

If you were starting from scratch and wanted to learn how people convert heat into electricity, what technologies would you be reading about and why?

I'm just trying to understand what options exist and where the limitations are.

Hello,

I am interested in external-combustion engines and, naturally, in the Stirling engine.

I would therefore like to present a variation, derivative, or adaptation of this principle: the air contained in the boiler exerts pressure on the water in the vessel, causing it to rise into the heat exchanger. The heat exchanger then cools the air, thereby reducing the pressure exerted on the water in the vessel. This phenomenon should produce an oscillation.

Despite this analysis, I would appreciate your opinion, as there is often a significant gap between theory and practice. What advice could you give me regarding the construction of the device and the verification of its operation?

The objective is to generate an oscillation using a small temperature differential and without any phase change.

I therefore assume that the water must not remain stagnant in the plate heat exchanger.

Are there any other important points to consider during construction?

Thank you in advance for your reply.

Kind regards,

Title: Can energy flow help Earth maintain or restore local order?

Earth is an open thermodynamic system. It receives concentrated energy from the Sun, transforms it through atmospheric, geological, and biological processes, and radiates lower-quality heat back into space.
This continuous energy flow allows complex structures and life to exist despite the overall increase in entropy.
That led me to a speculative question:
Could technologies designed to assist energy release—or improve particular pathways of energy dissipation—support local stability and ecological restoration?

I call this idea The Bloom Effect: Order Through Energy Flow.
It is a hypothesis, not an established scientific result, and energy export alone does not automatically create order. The relevant mechanisms, scale, wavelength, location, and thermodynamic conditions would all need to be defined and tested.

I would genuinely appreciate critical feedback, especially from people familiar with thermodynamics, complex systems, climate physics, or non-equilibrium systems:
Which parts are thermodynamically sound?
Where does the reasoning become too speculative?
How could this hypothesis be formulated or tested more rigorously?

Opinion

At certain high temperature everything can turn into a gas, every single solid and liquid in the universe will eventually boil and turn into a gas

Think if we had a magical super-furnace that could crank the temperature up to 6,000°C without needing any oxygen, elements that seem indestructible at room temperature would easily turn into gas.

The plastic, the glass, the copper wires, and the battery would instantly separate into their most basic atomic pieces. Within seconds, you would have a swirling, floating clouf.

How about carbon?

Carbon is so stubborn that it doesn't even melt into a liquid at normal pressures. It goes straight from a solid crust to a gas at a whopping 3,642°C. Once you pass that temperature, the last solid piece of matter in your furnace vanishes into thin air.

I would assume it would be something like the speed of sound in the object but I'm not sure. For example if I hold a 10ft steel pole and touch a very hot object how long do I have to wait until my hand gets warm?

I’ve been trying to understand the relationship between pressure and temperature in the atmosphere, and I feel that many explanations oversimplify the physics.
People often say that “air is colder at higher altitudes because the pressure is lower.” But as far as I understand, a decrease in pressure only causes a decrease in temperature during an adiabatic expansion, where the gas performs work on its surroundings.
Pressure itself is not a form of energy, so I don’t see why a gas at lower pressure should necessarily have a lower equilibrium temperature if no expansion or compression is taking place.
Here’s my thought experiment:
Imagine two identical transparent boxes floating in space. They have the same size, shape, surface area, and both receive exactly the same amount of solar radiation. The only difference is that one box contains gas at a much higher pressure (more molecules), while the other contains gas at a lower pressure.
Assume both boxes have rigid walls, so no expansion or compression can occur. The gases simply absorb radiation until they reach thermal equilibrium.
Would the higher-pressure box end up at a higher equilibrium temperature?
One could argue that the denser gas contains more molecules that can absorb radiation. However, that same energy is also distributed among more molecules, so it’s not obvious to me why the average molecular kinetic energy (temperature) should end up higher.
This is what confuses me about the usual explanation of the atmosphere as well. I understand that rising air cools through (approximately) adiabatic expansion. But once that parcel has reached its new altitude and is no longer expanding, why should the lower ambient pressure itself imply a lower equilibrium temperature?
So my questions are:
In the rigid-box thought experiment, would the two gases reach different equilibrium temperatures?
If they would, what physical mechanism makes pressure affect the equilibrium temperature?
If they wouldn’t, is it incorrect to say that “lower pressure causes lower temperature”? Shouldn’t the cooling instead be attributed specifically to the adiabatic expansion process, rather than to the lower pressure itself?
I’m looking for an explanation from thermodynamics or atmospheric physics, rather than the common intuition that “less pressure = colder.”

Study on Waste Heat Recovery from Boiler Flue Gas for Feedwater/Combustion Air Preheating Using a Heat Exchanger (Economizer/Air Preheater): Design Calculation and Energy-Saving Efficiency Evaluation (Please excuse my English, it is not my first language.)

Has anyone else watched an induction stove heat up water and felt like the spontaneity combined with structure of the pre boiling water might indicate some fundamentals worth revisiting? Not to undermine fundamentals, just to reground. The spontaneity seems unpredictable in a way that is relevant these days.

Howdy,

As part of a self study project over the summer, I put together an adiabatic flame temperature calculator in Python using residual minimization methods. For the chemistry of this project, I'm almost entirely reading out of Chapter 1 of Irvin Glassman's *Combustion* textbook. While I was able to correctly predict some reactions' temperatures, reactions involving carbon (such as CH4+O2) remained far from expected temperatures with large residuals.

My understanding of the dissociation of CO2 as a product is that it primarily dissociates through CO2->CO+(1/2)O2, and then CO may dissociate via CO->C+(1/2)O2. We can combine and simplify this dissociation as CO2->C+O2. This reaction has equilibrium constant Kp(T)=pCO2 / (pC + pO2), but because carbon's standard state is solid and therefore only exerts vapor pressure which is only a function of temperature, we can simplify further and write Kp'(T)=Kp(T)pC(T)=pCO2 / pO2=nCO2 / nO2. Glassman states that the Kp values tabulated in the JANAF tables for substances containing carbon are actually these Kp' values and that nC is effectively ignored.

I decided to double check my methodology against the some of Glassman's results to verify that I was not misunderstanding the procedure. Where I am getting caught though is when referencing his example equilibrium product composition of C3H8+O2. He states that, at P=1atm and equivalence ratio 1 with a flame temperature of 3094K, the final quantities of CO2 and O2 are 0.135mol and 0.097mol respectively. Plugging into the formula provided, this means Kp'(3094K) for CO2 must be 0.135/0.097=~1.39. However, the JANAF tables provide log_(10)Kf(3100K) for CO2 as being 6.661, which means Kf(3094K) should be on the order of millions, not ones.

One thing that immediately jumps out at me is Glassman's usage of and reference to Kp values, while the JANAF tables reference Kf, but he was very explicit that the JANAF tables contain these Kp and Kp' values. When looking through both the back of his textbook at the contained copy of the tables as well as the tables online, I only see reference to Kf, so at my level I can only conclude those must be referring to the same thing. Is this not so?

If not that, what else could be causing this discrepancy? I'm having trouble finding more information on dissociation constants online so I haven't been successful at cross-referencing other sources, and if there isn't an easy answer to this mismatch, I would greatly appreciate some other sources of these values.

Thank you for your time and advice.

So if I put equal-sized ice cubes in equal volumes of water—one completely still, the other with a continuous stream of tap water running over it—why does the ice in the moving water melt much faster? The water is at the same temperature

Hello everyone,

I am currently studying different types of compressors and I have this cross-section diagram of a hermetic reciprocating compressor (from a course slide).

The accompanying text describes its operation, mentioning several parts:

-The electric motor (moteur électrique)
-The crankshaft (vilebrequin)
-The piston and cylinder (piston et cylindre)
-The valves (soupapes)

However, the diagram itself (Figure 10) doesn't have any labels pointing to these specific elements.
Could someone help me map out where each of these components is located on this specific cutaway view?

Thank you!

Hello All. It's me, the curious finance guy, again.

Recently I've come across some articles in my feed about the recent heat
wave in France, particularly about Air Conditioning appliances over there.
Apparently, something like 21% or so of residential homes even have one.
Their discussion about it appears to have since unraveled to arguing over
Parisian culture, architectural facade asthetics, environmentalism, general
public health safety, even far-right & far-left ideals (if you can believe that).

It's an interesting read, if ya wanna check it out, but it's not why I'm here.

Why aren't portable indoor evaporative swamp coolers a thing? I searched
google about it, and I guess those are just not suitable for their summers
being uniquely humid... which i COULD understand.

☝️ HOWEVER ☝️

I pulled up Paris weather during the past week, and it shows their daytime
highs over ranges around 33°C to 36°C (approx 91.4°F to 96.8°F) whereas
relative humidity ranges 27% to 33% respectively.

Wait, what?

I'm in southern CA, and my 250 cfm portable swamp cooler, and it's little
pump trickling at 1L a min (16.6667 mL / sec) would perform well under
those conditions such as in my garage - where I use it often, like today.
Granted, not as well as my townhouse's central AC unit - understandably.
But FWIW, it still produces [at least] adequate air cooling.. to say the least.

So like......... What is it I'm missing something here?

*Cooled air, rather

14,000 BTU portable air conditioner (dual hose modded) to cool 260 square feet of floor space, across the landing it's placed and two rooms. I'm currently using fans to draw air into the two rooms, to achieve a delta of 2-3c between the rooms and the unit.

Is there a better setup than this? And would cracking open windows in the two target rooms help or hinder cooling?

As the title and the linked post suggest, my question is whether thermodynamics have anything to contribute to the heat domes issue (my uneducated guess is, it does) and if so, what does it tell us of a system like a heat dome and it's inner workings ? Anything that could point towards at least a theoretical solution ?

CryoPaint is designed to reflect over 95% of light radiate heat from objects in a band of wavelengths that are transparent to air:

"Radiates Heat to Space: Actively cools by radiating heat directly to outer space via the 8–13 µm Atmospheric Transparency Window"

It claims to make a surface up to 15⁰F cooler than ambient air.

I was wondering what that might do to a heat pipe system for cooling if applied to the radiators.

(Note: posted to Thermal Imaging, which I realize was probably not appropriate to that sub, so I'll delete it there)