Date: Sun, 30 Jun 2002 01:47:30 +0100
Reply-To: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Subject: Re: Humid Air - More Comments and a bit of theory
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I thought that enthalpy would come into it somewhere - honest I did...
... but what we need are some graphs of the total heat transfer capacity of
a radiator matrix, at a given International Standard Atmosphere, for a range
of humidities to establish whether the curves increase or decrease. Let's
assome a dry surface too for now, with no water droplets around, nor thin
films. Any takers?
... no doubt this was all done many years ago by the aeronautical boys, I
suspect in the 30's (maybe by Rolls Royce having problems cooling their
massive 'R' engines for the Schneider trophy races, even resorting to
condensation surface cooling, a nightmarish engineering undertaking that
ultimately failed) or certainly in the early 40's, maybe during development
of the North American Mustang's underslung radiator system - the first to
actually achieve a 'net thrust gain' from an aero-engine cooling system -
making the Merlin engined P51 developed at Rolls Royce Hucknall the most
fuel efficient, if not quite the fastest piston engined fighter ever made.
A little research shows that its all down to a few Non-Dimensional Numbers,
Nusselt No., Reynolds No. and Prandtl No., certainly up to M 0.2, something
most vanagons would have trouble achieving. Velocity dependency is roughly
linear, implying that as we speed up, the problem is exacerbated. Power
required to overcome wind drag goes up as the square of speed, yet the
amount of heat that can be dissipated does not, requiring a radiator system
that is oversized for cruising speeds, although not for very low speeds or
idling without fan assistance. Hill climbing just adds a little more
complexity, again mitigating towards a larger or better radiator system.
These effects can be observed, particularly when the cooling system or
engine is not in good shape. When does overheating tend to occur? Idling in
traffic and at high speed or medium speed, high load hill climbing. At
nominal speeds, say 50-60 mph on a flat road, everything is matched quite
well. Maintain high speeds for long enough, and I've seen oil temperatures
(even on cars equipped with oil coolers) and sometimes coolant temps rise to
dangerous levels. Idle for long periods in traffic with anything wrong with
the cooling and again.
The best technique for economy is also the best technique for keeping heat
out of the coolant e.g. trying to get home with an overheating engine:
Drive nominally at cruising speeds where the engine is at peak torque
(minimum specific consumption) in top gear, using a light throttle, e.g.
usually 50 - 60, but not too slow. Accelerate and run down hills faster, DO
NOT hold back and go on the overrun, but gradually reduce power as the road
starts to climb going down the gears earlier rather than later, arriving at
the top of the incline with a speed below your level road crusing speed.
This is known as constant energy driving, and where a series of hills are
encountered involves a continual exchange of potential energy for kinetic
energy and vice versa. With a heavy, but low drag vehicle, the downslope
speed can be quite high for optimum efficiency, all that k.e. being slowly
converted to p.e. during the climb. With a draggy vanagon the speed
differences between trough and peak will be much less, but the idea is worth
knowing, in fact its intuitive and just plain common sense, backed up by a
little theory.
I have done this sometimes for 50 or more miles, noticing the same guy in a
fully loaded car overtaking me going up hills, whilst I've overtaken them
again down the next, time and time again. Guess who's using more fuel and
putting more heat into their cooling system? Not me. He was what are
euphemistically called 'Rover' drivers over here :-), abusing their power
privelege as well as their wallets, and sticking to the 70mph speed limit up
hill and down dale.
Of course I wouldn't dare mention the DANGEROUS practice of turning the
engine off going down long hills, but sometimes needs must..., not for fuel
economy, but when a head gasket's gone. No, guv, never done it... brake
servo wouldn't be working, but Europhiles are used to heavy brake pedals
(the Beetle, the Morris 1000, both abs. without feel, unless pushing a block
of concrete is 'feel') and I don't believe any VW vanagon (Transporter)
types have power racks here either, they're considered quite light on the
steering as standard, compared to many early FWD cars.
Clive Smith
'89 2.1 DJ engined Syncro Transporter panelvan
----- Original Message -----
From: "Keith Hughes" <keithahughes@QWEST.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Saturday, June 29, 2002 5:58 PM
Subject: Re: Humid Air - Some Comments
> Well, perhaps a bit more clarification. The evaporation of
> non-atomized water is not, strictly speaking, the only mechanism
> for moist air enthalpy change.
>
> The enthalpy (or energy content) of moist air at pressure Pb,
> temperature t (°C) and mixing ratio r (g H20 / kg dry air) is
> defined by:
>
> h [kJ / kg moist] = 1.00464 t + 0.001846 r x t + 2.5 r
>
> Note: by convention, the enthalpy of dry air ( r = 0 ) at 0°C is
> equal to zero. Negative values of enthalpy are actually possible
> and indicate that the energy content of the air / vapor mixture is
> less than the energy content of dry air at 0°C.
>
> From this it can clearly be seen that as the water content (truly
> gaseous water) increases, the enthalpy of the resulting mixture
> increases. That is, it contains (or can contain) a greater amount
> of heat. So, *for any given pressure*, the higher the humidity, the
> higher the heat transfer capacity will be. However, as has been
> said, the higher the humidity, the less dense the air. So, there
> are a couple of competing things happening here;
>
> 1. As the humidity increases, the enthalpy, and heat transfer
> efficiency, increases for any given pressure.
>
> 2. As the humidity increases, the density (and hence pressure)
> decreases.
>
> The enthalpy changes can be readily obtained from the equation
> above, or using a standard psychrometric chart. The pressure change
> is not as easy to calculate, since unfortunately
>
> PV=nRT applies to ideal gases only. And, while saturated steam can
> *almost* be considered ideal, water vapor cannot. Primarily due to
> the issues Frank raised about non-atomized clusters, and
> combinability issues.
>
> Keith Hughes
> '86 Westy Tiico "Marvin"
>
>
>
>
> > Date: Fri, 28 Jun 2002 23:32:29 +0100
> > From: Clive Smith <clive.harman-smith@NTLWORLD.COM>
> > Subject: Re: Humid Air - Some Comments
> >
> > Absolutely wonderful
> >
> > And talking of wonderment, I wonder if its the last word?
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