Hot answers tagged

9

The concept of what you're doing is sound, and as Russell McMahon notes the efficiency gains could be significant enough to justify the change. I'd strongly suggest that you consider adding a ramp to the back edge as well. Drag force is very sensitive to the downstream (rear) end of a body as well You get some positive pressure at the front of the vehicle, ...


8

You need to consider the density of the air, which varies with temperature and air pressure. At 15 degrees Celsius, at sea level, the density of air is 1.225 ${kg}/{m^3}$. The table here gives air densities at 5 degree intervals. Now density is mass divided by volume, ${\rho} = m/v$ Hence, to get the volume flow rate (in ${m}^3/s$), for a know mass flow ...


8

Silica gel is the standard for most purposes. It's the desiccant you find inside those paper sachets packed with new electronic equipment. It consists of tiny beads of silicon dioxide that are nanometres across (so that a little material has a lot of surface area), and although it's called a gel, the material itself is hard at room temperature and pressure....


7

This phenomenon is known as cavitation for pumping liquids - i.e. for anything that can change phase. If you are pumping gas, then the scenario really does come from choked flow. Let's say you've got a compressor and you're running it as a vacuum pump to evacuate a tank - at some point the tank's pressure on the inside is so low, you can't pump any more ...


7

The common pipe threads that are used in buildings for water and gas are tapered threads. The thread is cut on the cone rather than cylinder. In the US, these threads are called National Pipe Thread Tapered (or NPTT1, or simply NPT). They don't seal metal-to-metal. Thread seal tape is wound onto the male thread before it's screwed in. When the tapered ...


6

Are you bound to the shape/configuration of part you constructed in your question? If not, a simple 3-way pneumatic solenoid valve would make this very simple and leak proof(relatively speaking). They are inexpensive too.(eBay) You would have an input and the valve will allow flow through one outlet and then energize the solenoid and you will get flow ...


6

You are describing a very interesting engineering problem. The requirement of a turbine is to convert fluid-energy into mechanical energy over a range conditions. Since the flow around the airfoils is governed by the (elliptic) Navier-Stockes-Equations, an easy answer to your question is not possible in general. Only in certain edge cases it is possible ...


6

Quick and dirty CFD simulation of your problem using ANSYS Fluent 14.5: I used a 2D duct, 8" x 4" with a 45-degree angle going from the inlet pipes to the main chamber. Assuming 3 liters per hour flow through a half-inch diameter pipe gave me an inlet velocity of 6.6 mm/s. Air enters from the left and exits through the right. Inlet and outlet were set to ...


5

HVAC-Systems usually give a Volume-Flow-Rate, hence $m^3/s$. If you need Mass-Flow-Rate ($kg/s$) you simply need to multiply with the density ($\rho$) of the fluid. The density can be calculated using the ideal gas law (see 1): $$ \rho_s = \frac{p_s}{R T_s}$$ Please observe that you need static values for the pressure ($p_s$, see 2) and temperature ($T_s$, ...


5

What you want is called a heat exchanger. Imagine two long air tubes with a thin wall between them. The air exiting the house travels in one tube, and entering air in the other, but in opposite directions. Over the length of the tubes, heat transfers thru the thin wall. Ideally, by the time the house air gets to the far end where the outside air comes in,...


5

The fundamental problem is that hot air must leave the room and cold air must enter it, both at the same time through the same single opening. The conflicting forces will mostly cancel each other out, resulting in very little actual exchange of air. What you really need is old technology. Double hung windows are what our ancestors used. The bottom sash ...


5

In the Fluid Dynamics community about 40 years ago, the group was primarily divided into experimentalists and theorists. However, at that time CFD was quite new, had to be run on expensive supercomputers, and untrusted. It was quite common that a theorist or experimentalist would at best discount the results of the CFD, while others may totally disregard ...


5

In addition to tapered threads for creating seals, there are Compression Fittings which use a matching male and female mating low angle taper to create a sealing surface. When properly torqued, they rely on elastic deformation to "squish" the mating surfaces together to create a seal. These are re-usable. Crush washers are used in a similar fashion to ...


5

The short answer is that they don't. Choking is not a bad thing. For a gas flow regulator, it's actually a good thing, because under choking conditions, the mass-flow rate is no longer dependent on the downstream pressure. The mass-flow rate is still dependent on the upstream pressure and the size of the aperture connecting the high and low-pressure sides. ...


5

The capacity of an evaporative cooler depends on the relative humidity. The lowest theoretical temperature that the cooler can reach is the wet bulb temperature. The wikipedia article has some discussion on the design and shows how it works with psychometric charts. For the design you'd look at a 'typical' hot day, compare wet bulb to dry bulb for that day ...


4

CFD I ran this situation through a couple of simple 2D CFD models. As you can see from the pictures below, one way to keep debris from gathering in this area is to increase the velocity of wind in the corner. To do this, you need to place an obstacle that will direct the wind into the corner. Original Situation: Obstacle Added: Even with this, there will ...


4

It's a trade-off between the simplicity of the sensor and how comprehensive the reported data is. The locked rotor sensor only reports whether or not the motor is spinning, not how fast it's spinning or how much resistance is on the fan. It is a very simple signal though, it's either on or off. On the other hand, the tachometer signal is a bit more ...


4

You need to push air through the turbine at 11 m/s, or just above. That's what the rated windspeed means: it's the lowest windspeed at which the turbine hits rated power, at standard atmospheric pressure. The volume of air you need is determined by the area of the rotor. Let's say it's $\alpha m^2$. So you know you need to push through $11\alpha m^3/s$. So ...


4

Constant entropy doesn't mean constant temperature, the gas flow through nozzle is subject to change in pressure and velocity (continuity equation) due to the variation in cross section, and being compressible the change in pressure will be reflected as change in temperature and density. That being said, since the flow is isentropic and there will be no ...


4

The sun is heating mode. When the room temperature reaches the set temperature, the air conditioner stops operating until the temperature falls below the set temperature and the starts operating again. When in heating mode, the air conditioner does not cool. This setting is used during cold weather periods, such as in winter. The snowflake is cooling mode. ...


4

The equivalent of a capacitor in terms of pneumatics would be a chamber such as a metal sphere the size of a tennis ball. It would be necessary to have an external vacuum pump to evacuate the air prior to the quick release. Auto tire houses use the reverse concept to seat a new tire on the rim. A cylindrical tank is loaded to substantial pressure by a ...


4

Almost anything can be in those pedestals, but the more common inhabitant would be cable television related. The smaller ones are for subscriber taps, while the larger ones contain amplifiers or interface devices. Fiber optic cables are used for longer distances, while there are interface devices to convert from fiber to coaxial feeder lines to the ...


3

Making a duct that flares along an exponential curve to 2 feet (60cm) in diameter would create a larger duct that won't solve your problem. It would be an expensive duct to make (think tuba bell), and if you focus on the flare instead of the overall dimensions, then the duct might come down too low and you won't have enough space. Keep in mind that a ...


3

The resistance against the fan is influenced by the total resistance of the pipe down wind of of the fan (in additional to the resistance on the inlet side). The total resistance is calculated from the skin friction, diameter and length with equivalent lengths added for bends/turns, joints and the type of plenum used on the outlet. When you add length to ...


3

What you are describing is known as "choked flow". I am only familiar with choked flow in a gas, however a brief wikipedia snoop shows that liquids are limited by a different phenomenon, which makes physical sense if you understand the mechanism of choked flow in a gas. In its simplest form, it depends only on minimum cross sectional area and fluid ...


3

As I understand it, you would like to develop an equation for the flow rate on the exit side of the HEPA filter based on the static pressure the fan produces. The formula for the pressure versus flow rate fan curve will be unique to every fan. Influencing factors will be whether the fan is axial or centrifugal and for centrifugal fans, whether the fan ...


3

For simple ventilation, opening a single window may not mean a lot of circulation. Air has to come in and out of the same window, so it just leads to eddies and turbulence next to the window, with conduction doing most of the work. Opening two windows allows a pathway for the air to flow in and out, and allows the air to mix in, which creates an open heat ...


3

Taking the data you supplied at face value, a relatively simple solution is for the dryer to measure either the exhaust air flow or back pressure, and refuse to operate when out of spec. Sensing these parameters can be done cheaply with today's electronics, especially considering such dryers already have a microcontroller in them to do the sequencing, run ...


3

Stress on panel mounts needs considering. This can be greatly reduced by a "ramp" that fully protects the panel from air impact in the forward direction. Flat plate drag for a panel will be <= classic drag equation result. $$P_{lost}= D \cdot V $$ = power required to accelerate all opposing air out of the path of the object from rest to velocity $V$ ...


3

As you say, flaps on aircraft wings increase lift at the expense of also increasing drag. This also means that the minimum speed to get lift is lower. These tradeoffs is why airplanes don't fly with flaps down all the time, only during takeoff and landing when the extended low speed range of lift is really useful. To get a quick intuitive idea of why ...


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