Mixing two different liquids at different temperatures

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Discussion Overview

The discussion revolves around the mixing of two different liquids at varying temperatures, specifically focusing on the dynamics of temperature and concentration equilibrium. Participants explore the implications of thermal and molecular diffusivity, particularly in the context of injecting water into a concentrated salt solution at high temperature.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that when mixing two liquids at different temperatures, both temperature and concentration will initially be uneven but will approach equilibrium over time.
  • Another participant mentions that the rate at which temperature and concentration reach equilibrium can be compared using the Lewis Number, which relates thermal diffusivity to molecular diffusivity.
  • A participant raises a specific scenario involving injecting water into a concentrated salt solution at 250°C, questioning whether the water will boil before being absorbed or vice versa.
  • It is suggested that modeling the mixing process, potentially through simulations of striated slabs of water and salt water, could provide insights into the dynamics of the situation.
  • One participant expresses that the problem may involve complexities related to turbulent mixing, which is an area of ongoing research in fluid dynamics.

Areas of Agreement / Disagreement

Participants generally agree on the relevance of the Lewis Number and the importance of initial conditions, but the discussion remains unresolved regarding the specific dynamics of temperature and concentration equilibrium in the given scenario.

Contextual Notes

The discussion highlights the dependence on initial conditions, such as the starting temperatures and concentrations of the liquids involved, which may influence the outcomes of the mixing process.

RobertHutchison
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When I pour two different liquids at different temperatures, into the same container, the temperature will be uneven at the start but then will approach equilibrium. Similarly, the concentrations of each liquid will be uneven but when mixing is complete, there will be the same concentrations throughout the liquid. I am wondering whether the temperature will reach equilibrium first or whether the concentrations will reach equilibrium first.
A bit of a tricky one I think.
 
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P.S. the liquids are completely miscible with each other.
 
If you're stirring the system, both will happen very fast. In any case, to ascertain which will happen faster, you need to compare the thermal diffusivity with the molecular diffusivity. The ratio of the thermal diffusivity to the mass diffusivity is called the Lewis Number (a dimensionless group).

Chet
 
Thanks Chet.

I have quickly looked at the Lewis number and it is certainly relevant to the problem. I will chase this through. The particular problem I am investigating is injecting water into a concentrated salt solution at 250C. I am wanting to find out whether the water will boil before it is absorbed or it will be absorbed before it boils. Any suggestions?

Robert
 
Of course the Lewis number tells you a lot about the problem. It it will also depend on the initial conditions, i.e. What is the temperature of each liquid at the beginning and what is(are) the initial concentration(s).
 
RobertHutchison said:
Thanks Chet.

I have quickly looked at the Lewis number and it is certainly relevant to the problem. I will chase this through. The particular problem I am investigating is injecting water into a concentrated salt solution at 250C. I am wanting to find out whether the water will boil before it is absorbed or it will be absorbed before it boils. Any suggestions?

Robert
Well, I think you can get a handle on it by doing some modeling. But, it would have to include some simulation of the mixing. Maybe you could start by modeling an array of alternate striated slabs of water and salt water. This is how I might begin to approach something like this.

Chet
 
Chestermiller said:
Well, I think you can get a handle on it by doing some modeling. But, it would have to include some simulation of the mixing. Maybe you could start by modeling an array of alternate striated slabs of water and salt water. This is how I might begin to approach something like this.

Chet

Getting any more detailed than that may be... problematic. This problem reeks of turbulent mixing in a jet, which is an active area of modern fluid dynamics research.
 
Thanks Chet and boneh3ad. Your advice has been very helpful.
 
boneh3ad said:
Of course the Lewis number tells you a lot about the problem. It it will also depend on the initial conditions, i.e. What is the temperature of each liquid at the beginning and what is(are) the initial concentration(s).
Off topic, but, I must say, your Dr. Strangelove is a spectacular choice of avatar. I doff my cap to you sir!
 
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