Using Pipe Curve Models

Last post we demonstrated a parabolic model for the pressure-flow relationship in a pipe, and showed a more robust way to represent the flow-pressure relationship.  In this post, we’ll discuss the application of both models.

Inverting the Pipe Curve

Fluid flow in a pipe, described by the pressure drop as a function of flow rate, is commonly modeled with a parabola: ΔP=a*Q² + b*Q + c.  However, it is sometimes very useful to have the inverse relationship: Q(ΔP).  In this post we’ll talk about a convenient and accurate way to model that inverse.

Mary, Mary, How does your dewpoint vary?

Today we are modifying the old nursery rhyme which inquires about Mary's garden  to consider typical fluctuations in dewpoint temperature.  Since the dewpoint depends on the total atmospheric pressure, and that changes from day-to-day, the dewpoint will change (for a given amount of moisture in the air) as the barometric pressure changes. We'll also look at how dewpoint might change with altitude.

Heat from Breath, part 2

In the last post we considered heat lost through breathing and determined that breathing air at 35 °F results in a heat loss of about 2 Watts for every liter/min of air flow.  Of course, this depends strongly on the temperature and relative humidity of the ambient air.  In this post, we’ll consider both the effect of the condition of the ambient air, and also how much of the heat is latent (due to moisture) and how much is sensible (due to temperature).

Heat from Breath

In an earlier post, we looked at how much water is lost through breathing.  Today, we’ll consider the amount of body heat lost through breathing.  Of course, depending on surroundings, air movement, and  how one is dressed, a lot of (or a little) heat might be lost through the skin, but we’re just considering the part that goes out with your breath.

Buoyancy

I used to read to my kids from a popular children's book which featured a curious monkey who got into trouble by grabbing a large bunch of helium balloons from a balloon salesman.  According to the storyline, the monkey was carried out over the city hanging from the balloons.  Now, I'm sure the illustrator for the book was deeply interested in thermodynamics (isn't everyone?) but may have been in too much hurry to calculate the appropriate number of balloons.  Or maybe artistic aesthetics overruled thermodynamic accuracy.  In any case, today we'll go back and look at the situation in more depth.

The Cyclic Relation

The cyclic relation is a result from calculus that is often used in thermodynamics to generate relations between properties.  Today we’ll talk about the cyclic relation and demonstrate it from tabulated steam table properties.

Moisture and the Speed of Sound

Many of us first learned about the speed of sound as children when we were taught that counting the seconds between the flash of lightning and the sound of the thunder provided an estimate of the distance to the lightning strike (around 5 seconds per mile).  Later, perhaps, we learned that the speed of sound is calculated by sqrt(k*R*T) where k is the specific heat ratio, R is the gas constant for a particular gas, and T is the temperature.  Obviously, the speed of sound varies with the temperature, but it also varies with the makeup of the air (through k and R).  So, does humidity have a significant effect on the speed of sound?  We’ll explore that question today.

Moisture and Altitude

Psychrometric charts are very useful for determining properties of moist air, but it is important to remember that each chart is prepared for a given atmospheric pressure.  Of course, it is easy to find charts created for specific elevations, but it is sometimes hard to visualize the variations in properties with elevation.  In this post, we’ll look at the effect of elevation on the moisture content of air.

Visualizing Thunderstorm Heat

When water vapor in the atmosphere condenses into liquid water we first get clouds, then rainfall.  But water vapor only condenses when heat is taken away from it.  So I got to wondering how much heat movement is involved in a rainstorm.

A Cold Snap, Part 2

In the last post, we talked about the ground temperature during a cold snap.  Today, we’ll look at the same situation from two different viewpoints.

A Cold Snap

Following a series of relatively warm days, we had a sudden (over a period of a few hours) drop in temperature and things then stayed very cold for several days.  This led me to wonder how fast the ground would cool under such circumstances.

For the purposes of getting a simple, quick feel for the effect of the air temperature change, we’ll look at a somewhat simplified version of the actual events.  Of course this result won’t exactly match what happened in real life, but it will provide a reasonable estimate.