Can someone check my work, please?
Can someone check my work, please?
(OP)
This is a bit out of my area:
For air @ 80F:
d = 1.5mm (dia lumen)
L = 15cm (lumen length)
mu = 2.07 Pa.s
rho = 0.9996 kg.m^3
V = 1 m/s
Re = (rho * V * d)/ mu
Re = 0.00072 - is that really right?
Q = (V * pi * d * d)/4
Q = 1.77E-06 cc/min
That would mean a pressure loss of:
delatP = (128 * mu * L * Q)/ Pi * d * d * d * d)
= 4.42E+6 Pa or 640 psi - something seems off...but, I did say this is not my comfort zone.
Thanks for any help you can give me.
Kirk
For air @ 80F:
d = 1.5mm (dia lumen)
L = 15cm (lumen length)
mu = 2.07 Pa.s
rho = 0.9996 kg.m^3
V = 1 m/s
Re = (rho * V * d)/ mu
Re = 0.00072 - is that really right?
Q = (V * pi * d * d)/4
Q = 1.77E-06 cc/min
That would mean a pressure loss of:
delatP = (128 * mu * L * Q)/ Pi * d * d * d * d)
= 4.42E+6 Pa or 640 psi - something seems off...but, I did say this is not my comfort zone.
Thanks for any help you can give me.
Kirk
Kirk B Olson
Senior Principal Engineer
HEII, Inc.





RE: Can someone check my work, please?
Well if your formula are correct then your number crunching is also correct.
regards desertfox
RE: Can someone check my work, please?
RE: Can someone check my work, please?
Kirk
Kirk B Olson
Senior Principal Engineer
HEII, Inc.
RE: Can someone check my work, please?
Kirk
Kirk B Olson
Senior Principal Engineer
HEII, Inc.
RE: Can someone check my work, please?
The units for your Q (flow rate) are wrong. The value of 1.77E-06 should have the units m3/s. If you want it in cc/min the value is 106. Assuming velocity of 1 m/s is correct?
The Reynolds number (if visc = 2.07E-5 Pa.s) is 72. This gives you laminar flow and fortunately you don't have to worry about the surface roughness to calculate the friction factor (which is implicit in the equation you used).
The pressure drop should be about 0.044 kPa (=44Pa =0.0064PSI =4.5mmH2O). This ignores entrance and exit effects, which will be very small at these low velocities.
If you plug the correct values into your formula you should get the correct answer (I have not checked your formula for consistency of units). At these low pressure drops you can safely ignore the complications of compressible flow - from your density and temperature I infer that you are working close to atmospheric pressure.
RE: Can someone check my work, please?
You are using SI units. Therefor you'd need to get your lengths all into metres: 1mm = 0.001 m. the Pa.s is the correct unit of measurement for viscosity. The air viscosity is given by: AIR_VISC = -1.555E-14 * T ^ 3 + 0.000000000095782 * T ^ 2 + 0.000000037604 * T - 0.0000034484 (T in K). For that density, the air temperature is around 345K and the viscosity then should be around 2.00E-5 Pa.s. The Re = 75 i.e. laminar flow. The volumetric flow MUST be m^3/s = 1.76715E-06 m^3/s. Hence, the pressure drop is (using the formula that you gave) equal to 42.6 Pa.
remove.marius@mailbox.co.za
RE: Can someone check my work, please?
remove.marius@mailbox.co.za
RE: Can someone check my work, please?
I hope the members of this forum don't get suspicious and think that we South Africans are conspiring together?!?
On a more serious note, I firmly believe that South African engineers have a huge advantage in that we are forced to work in SI units (mostly), but so many of our information sources are in US Customary or old metric units. This makes us so much more aware of the potential problems with units - although I have to admit I do still get tripped up now and then.
Harvey
RE: Can someone check my work, please?
Nice to see a familiar face...
Marius
remove.marius@mailbox.co.za
RE: Can someone check my work, please?
The other benefit of using such a program is that dimensional analysis also comes automatically; if your final units are not correct, you'll know that something got inputted incorrectly.
TTFN
RE: Can someone check my work, please?
density: 1.1774 kg/m3
absolute viscosity: 1.983*10-5 kg/(m.s) = 1.983*10-5 Pa.s
The values given by gt5pilot seem to better suit a temperature of 170 deg F rather than 80 deg F. Any comment on the accuracy of my source ?
RE: Can someone check my work, please?
Aren't you the people who caused the first known traffic accident on Mars? ;>
M
remove.marius@mailbox.co.za
RE: Can someone check my work, please?
Additionally, the failure of the interface was merely the tip of the iceberg. There were many occasions during that mission that the controllers were given indications that the probe was misbehaving and all the data was ignored until the probe was lost.
TTFN
RE: Can someone check my work, please?
Let me quickly see if I can get this forum activated: When are you Americans going to do do away with the dollars (or was it punds) per square thumb (or inches or something) system and get stuck into REAL units of measurement i.e metres and pascals and so on. I honestly do not know how any civilised nation can go on engineering in 12.34556 whatevers to 1.808748 whatever elses. Lesson 1 in SI: 1000 milimetres = 1.000 metre. EASY!!! see?
remove.marius@mailbox.co.za
RE: Can someone check my work, please?
Metric countries - please tape up your glass houses prior to tossing stones.
David
RE: Can someone check my work, please?
RE: Can someone check my work, please?
However, we now have computers and programs that can do the conversions, so the argument is quite moot.
Once "c" is entered as 2.99792458*10^8 m/s, it matters not whether you use it that way or use 1.80261750*10^12 furlong/fortnight (courtesy of v12, which includes both as built-in units).
So, the notion that we "must" use SI units, because it's somehow "easier" is moot, because the software has made that argument irrelevant and outdated.
Moreover, since real things are rarely that neat, 1 atm is 1.0132*10^5 Pa, so conversions are inevitable, no matter what unit system you use.
TTFN
RE: Can someone check my work, please?
On the subject of units, who cares if imperial or metric is the superior choice. I think this is a personal decision and is a reflection of an individual comfort zone. Ultimately, the answer should be EQUAL following system unit conversion. Just remember that mass is measured in "slugs" in the imperial, not poundal, pounds or otherwise.
Engineers should be equally comfortable working through the mathematical physics in either system. I can just hear Issac Newton groaning....
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Can someone check my work, please?
Firstly, the system with which you are most familiar will always seem the "best"
Secondly, anyone who has seriously switched over to SI will never want to go back. If you want to go back, you have not made the switch completely.
Cockroach said "Just remember that mass is measured in slugs in the imperial system, not poundal, pounds or otherwise". I wonder how many people using the Imperial system on a daily basis know that this is why you have to include the gc factor in some equations?
It is great that computers can take care of the units for us in so many instances, but like anything else that the computer is doing for you it is best if you have the understanding to do it by hand if you had the time.
RE: Can someone check my work, please?
It seems as though the question that I asked has resulted in a quasi-political debate on the "best" unit...I wish that is what I had asked originally, then I would have felt I made a contribution to this board...
[THREAD HIJACK OFF...]
Kirk
Kirk B Olson
Senior Principal Engineer
HEII, Inc.
RE: Can someone check my work, please?
It's equivalent to arguing about whether you should use "*" or "x" to signify multiplication or arguing about whether a 5-scale or 9-scale sliderule is better. It makes no difference so long as the job is done accurately and in a timely manner.
TTFN
RE: Can someone check my work, please?
Patricia Lougheed
Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.
RE: Can someone check my work, please?
It allows you to directly calculate BTU/hr-J, without any secondary conversions or analyses.
It's no different that if you entered a bunch of numbers with different exponents into a calculator. You assume and expect that the calculator will figure out the correct exponents in the final calculation. There's no need to verify the scale, as you would have needed to do with sliderules.
Automatic dimensioning is simply the next step in the evolution of math tools. I never have to worry about the units, because MAthcad treats the units in a similar fashion as calculator would with exponents. I have not used or needed to do any unit conversions manually or with some other program in many years.
The analyses I run are usually based on customer, i.e., the US military, specifications, which are often a mix of metric, English and anything the customer chooses or uses. It's not uncommon to find a requirement containing all of the following units:
m, yd, mi, nmi, um, kt, ft, in.
The only unit that wasn't included in Mathcad was the nautical mile, so that's permanently defined on my worksheet template. Calculate the lookdown angle as atan(alt(in ft)/rng(in nmi), and Mathcad merrily cranks out the angle, as it should. Diffraction blur diameter of 2.44lambda/aperture can be entered with lambda in angstroms and aperture in furlongs just as easily as entering lambda in microns and aperture in inches.
Therefore, it's completely irrelevant what units my customer uses and it makes no difference in the data entry or calculation process.
This is the next step in tool evolution and probably why there may not be any further movement toward SI units, as there no mechanical or calculation advantage to do so.
TTFN
RE: Can someone check my work, please?
Now I know why there is a search party on Mars looking for a banged-up rover...
remove.marius@mailbox.co.za
RE: Can someone check my work, please?
http://www.cnn.com/TECH/space/9911/10/orbiter.02/
: )
[ HIJACK STOP ]
RE: Can someone check my work, please?
- England is getting metric: millimeter by millimeter
- US is getting metric: inch by inch
Cheers,
CARF
[ HIJACK STOP ] ( Unless you wanna discuss football, but this maybe painfull for all of you ; )
RE: Can someone check my work, please?
Discuss.
;)
RE: Can someone check my work, please?
FURTHERMORE, the article fails to explain HOW the interface specification was not properly vetted by the design teams and the design reviews.
So, ultimately, the failure was a process failure, NOT because of using any particular units, since obviously, if both teams had used the same English units, the module would have worked correctly. Therefore, the lead sentence of the article was designed for sensationalism, rather than verisimilitude
from the MCO report:ftp://ftp.hq.nasa.gov/pub/pao/reports/1999/MCO_report.pdf
TTFN
RE: Can someone check my work, please?
A colleague of mine told me recently that your people (meaning aerospace) had accellerated a photon beyond c. Perhaps you are aware of this? From what he told me it seems that a photon possesses a certain duality, consisting of two equal parts with opposite spin (I think these are called bosons?). Anyhow, apparently these two particles will, when traveling along any plane, assume station with regard to each other.
Apparently (and please forgive me if this is total hogwash; I do not know any better) it is possible to bend one half of the photon into a detour. The other half continues straight on. Since the two must hold station on the straight line with respect to each other (is this a law?), the detoured particle is accellerated beyond c.
Could you kindly point me to the correct forum for this discussion.
Thanks
M
RE: Can someone check my work, please?
That's more in the realm of theoretical physics. Not sure if there is really a forum for that particular subject.
TTFN
RE: Can someone check my work, please?
If beyond c is possible we could travel in time, kill Napoleon before introducing the SI system, and I would be drinking a gallon of beer now !
Cheers,
CARF
RE: Can someone check my work, please?
http://www.lmnoeng.com/Flow/GasViscosity.htm
JMW
www.viscoanalyser.com
Eng-Tips: Pro bono publico, by engineers, for engineers.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Can someone check my work, please?
The tables provide data on density, viscosity, specific heat, thermal conductivity, diffusivity, and Prandtl number at amospheric pressure and at a wide range of temperatures.
RE: Can someone check my work, please?
Ultimately, I will have to model the heat transfer from the selected gas through human tissue and try to calculate the depth of neucrosis for varying temperatures and times.
Kirk
Kirk B Olson
Senior Principal Engineer
HEII, Inc.
RE: Can someone check my work, please?
TTFN
RE: Can someone check my work, please?
Gas does not have the heat capacity of water, but, it has some safety advantages...as well, it can be heated to higher temperatures that the current therapy temperatures (~90C) to shporten the therapy time. Helium is the most promising gas to date...
Kirk
Kirk B Olson
Senior Principal Engineer
HEII, Inc.
RE: Can someone check my work, please?
remove.marius@mailbox.co.za
RE: Can someone check my work, please?
Maybe something like an upside-down heat sink made from Teflon with an array of holes in the base. The fins would maintain a consistent spacing to the skin and the array of holes would more uniformly heat an area.
TTFN
RE: Can someone check my work, please?
That used to be a big problem, because, without debridement and/or sectioning, the observational results were very subjective and varied according to the skill and experience of the doctor. One of our abandoned projects was to try and correlate hyperspectral imagery with objective lab measurements.
TTFN
RE: Can someone check my work, please?
In the current systemm the fluid is introduced into a thin walled silicon balloon - we will use the same delivery system with our new fluid, gas. Yes, it is problematic - at best. Diffuser design for the gas is proving to be tightly coupled with the other parameters perviously mentioned to get even heat distribution in the balloon.
We actually disect the tissue, then stain it to determine the level of neucrosis.
I would be interested in hearing of your plights and challenges further...
Kirk
Kirk B Olson
Senior Principal Engineer
HEII, Inc.
RE: Can someone check my work, please?
Did you mean silicone? Silicon is thermally quite conductive and would probably have horrible uniformity. I think that even silicone would have the same problem, unless you had a VERY high gas velocity, otherwise, convection effects will result in lots of non-uniformity. You may need to have a stirring fan inside the balloon.
TTFN
RE: Can someone check my work, please?
Right now, we are running experiments using Helium and CO2 - we need to run quite high velocities/volumes, as you have thought would be necessary. Temperature uniformity is an issue for us - we have been desiging and experimenting with different types of diffusers to help minimize the problem. What we do find is that one particular diffuser will work better in smaller cavities, another in a larger cavity, etc. We have been doing a fair amount of modeling to find the best compromise and then trying to back up the models with experimentation.
Kirk
Kirk B Olson
Senior Principal Engineer
HEII, Inc.
RE: Can someone check my work, please?
TTFN
RE: Can someone check my work, please?
Kirk
Kirk B Olson
Senior Principal Engineer
HEII, Inc.