Homework free convection - Esteban Rodriguez Daza

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Homework	3	heat	transfer	2023-10	
	
	
1)	A	horizontal	tube	of	12.5-mm	dimeter	with	an	outer	surface	temperature	of	
240°C	is	placed	in	a	room	with	an	air	temperature	of	20°C.	Estimate	the	heat	
transfer	rate	per	unit	length	of	the	tube	due	to	free	convection.	
	
2)	A	40-cm	 -diameter,	110-cm-high	cylindrical	hot-water	 tank	 is	 located	 in	 the	
bathroom	of	a	house	maintained	at	20⁰C.	The	surface	temperature	of	the	tank	is	
measured	 to	 be	 44⁰C	 and	 its	 emissivity	 is	 0.4.	 Taking	 the	 surrounding	 surface	
temperature	to	be	also	20⁰C,	determine	the	rate	of	heat	loss	from	all	surfaces	of	
the	tank	by	natural	convection	and	radiation.	
	
3)	Air	at	-10⁰C	flows	at	10	m/s	over	the	roof	plate	of	two	5-m	length	rooms	whose	
air	is	at	T¥	=	20⁰C.	The	roof	plate	is	0.20-m	thick	concrete	(k	=	0.6	W/	m.	K).	a)	
determine	the	heat	 transfer	rate	 from	each	room	to	the	external	air	per	width-
room	unit	and	b)	now	assume	that	the	rooms	are	insulated	with	a	0.20-m	layer	of	
fiberglass	(k	=	0.043	W/m2.	K),	sticking	to	the	inside	surface	of	the	plate	concrete,	
and	determine	as	the	heat	transfer	rate	changes.		
	
4)	Water	enters	a	horizontal	thin-walled,	50-mm	diameter	long	tube	at	0.2	kg/s	
and	90°C.	The	tube	is	placed	in	a	room	with	quiet	air	at	10°C	.	
a)	Estimate	the	pipe	length	required	to	cool	the	water	to	30°C.	
b)	 Now	 estimate	 the	 water	 outlet	 temperature	 (Tm,o),	 the	 average	 surface	
temperature	of	the	tube	(Ts),	and	the	heat	transfer	rate	if	the	tube	length	is	8	m.