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hammercoupleMechanics

Feb 22, 2014 (3 years and 10 months ago)

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.לוחכ/םותכ/בוהצ עבצב הריאמ לשמל היזג לש הבהלה רשאכ בוריקב הרוטרפמטה המ תעדל שקבמ ינא

?הבושת עיצהל וא עדימ רוקמל יתוא תונפהל לכות םאה


ימדק ןדרי

http://www.natuurkunde.nl/vraagbaak/view.do?request.requestId=12745

http://utias.utoronto.ca/~groth/laminar_flames/AMRTempcompare.jpg


Op je bunsenbrand
er zal de ruisende blauwe vlam
het heetst zijn. Op bovenstaande afbeelding een
vlam die daar wel op lijkt. Het is wél een
methaanvlam, maar trek daaruit niet de conclusie
dat je schoolbrandertje ook 2100
-
2200 K kan
halen.

(Gronings) aardgas is geen zuiver
methaan,
dat bevat nog zo'n

20 massaprocent stikstof

%, en
dat moet ook mee opgewarmd worden. 1400
-
1500
K in practische situaties op het heetste punt in de
vlam is op school een goede prestatie.

איה לוחכ=םודאה עבצב הבהלה ,ןסנובה רעבמב
נה הנומתה .רתויב המחה
תקולח הארמ ל"
ןאתמ תבהל םנמא וז .וזכ הבהלב הרוטרפמטה

]ןאטוב אל ,רפס יתבב יעבט זגב םישמתשמ הפוריאב[
,
ל עיגהל ןתינש הזמ קיסהל ןיא לבא
-
2100
-
2200
K

םע
ליכמ הלא ,רוהט ןאתמ וניא יעבט זג .ירפס תיב רעבמ
כ
-

02
%

.הבהלה ךותב םמחל םיבייח ותואו ,ןקנח
וטרפמטה תואיצמב
,הילא עיגהל ןתינש תיברימה הר
איה ,ירפס תיב רעבמב

1400
-
1500 K
.

http://stores.ebay.ca/AVOGADROS
-
LAB
-
SUPPLY/How
-
Hot
-
is
-
a
-
Bunsen
-
Burner
-
Flame.html


How hot is a
Bunsen burner flame?


According to the North American COMBUSTION HANDBOOK, Vol 1, p. 12



Adiabatic flame temperature of propane/air is 3572
°F /

1967
°C

The answer to this very simple question may not be as simple as you may think.

So let’s take a look a
t some general facts that determine how hot a
propane

fueled,
Bunsen burner flame is?

Let’s begin with the combustion reaction itself.



C
3
H
8
+

5O
2

-------
> 3CO
2(g)
+

4H
2
O
(l)


Δ
H

=
-
2020 kJ/mol



Theoretical Combustion is the ideal combustion process during which a fuel is burned
completely. To achieve the maximum heat of combustion, for every 1 mole of propane
that enters the reaction 5 moles of oxygen must be available. A co
mplete combustion
occurs when all the carbon (C) has formed (CO
2
) molecules and all the hydrogen (H) has
formed (H
2
O).



1) Fuel rich mixtures, excess propane or insufficient quantities of oxygen will result in a
diminished temperature. If there are un
burned components in the exhaust gas such as C,
H
2
, CO the combustion process is incomplete. Un
-
reacted propane and a yellow flame
will result.



2) Fuel Lean mixtures, excess oxygen or insufficient quantities of propane will also result
in a diminished f
lame temperature. A short, sharp inner cone, deeper purple, hissing,
popping, unstable flame will result.



3) As the quantity of propane and oxygen reacting increases so does the amount of heat. A
larger burner flame is not hotter then a smaller burner
flame but it does provide more heat
given the same fuel
-
air mixture.



When consulting textbooks for the temperature of a propane flame one will discover that
there is a rather wide range of reported temperatures.



This variance can be due to a number of
factors.



1)


Instrumentation error: Measuring the temperature can effect

the flame temperature
and not all thermocouples are as accurate one would like.

2)


Where in the flame was the temperature

measured?

3)


Fuel flow is it laminar (smooth) or tu
rbulent? (bunsen burners use a laminar flow)

4)


Is the fuel premixed?

(fuel is premixed in the bunsen burner tube) The propane and
oxygen molecules must collide if they are to react.



Adiabatic Temperature

Flame temperatures are reported as adiabatic meaning that there is no loss of heat energy
to the environment or to external objects. This is a purely fictional scenario but
convenient when computing enthalpi
es of reactions. How much of the heat energy of the
flame is absorbed by the burner head or radiated out to the environment?





Heat Loss



Minerals and metals act as a heat sink. When placed in a flame they reduce the
temperature of the flame. Th
e amount of temperature change
Δ
T is a function of the
quantity of heat transferred, the specific heat of the sample and it’s mass. q = (specific
heat)(mass)(
Δ
T). Flame temperature can also be affected by the volume of the sample. As
the volume of the sa
mple increases, its surface area increases which promotes the radiation
of

energy to the environment.



So perhaps the question of how hot the bunsen burner flame is really isn’t clear
cut. However you would be on safe ground to assume that if the melting

point of
the sample metal is at or near 1500°C that an acorn size sample of the metal will
not melt under a Bunsen burner flame.








http://www.vernier.com/innovate/innovativeuse46.ht
ml

Flame Mapping


Figure 1

A thermocouple is one of the few temperature measuring devices that can be used to
measure the temperature of flames. Simply inse
rt the probe into the flame, and record the
temperature using your Vernier data
-
collection program. Typical student results are shown
in Figure 1. This activity could be used as an introduction to the Bunsen burner. Students
will learn to use a Bunsen burn
er more effectively by mapping the temperatures and
determining the hottest and coolest part of the flame, as well as the optimum conditions.